diff --git a/Makefile b/Makefile
index 957a57f..35b2367 100644
--- a/Makefile
+++ b/Makefile
@@ -60,6 +60,12 @@ override CPPFLAGS += -DFREESTANDING
 LIB_CFLAGS += -ffreestanding -nostdlib
 endif
 
+# Don't use this option except for testing; it isn't a stable interface.
+TEST_SUPPORT__DO_NOT_USE :=
+ifdef TEST_SUPPORT__DO_NOT_USE
+override CPPFLAGS += -DTEST_SUPPORT__DO_NOT_USE
+endif
+
 ##############################################################################
 
 PREFIX ?= /usr/local
diff --git a/lib/arm/cpu_features.c b/lib/arm/cpu_features.c
index 8d1facc..c4556a3 100644
--- a/lib/arm/cpu_features.c
+++ b/lib/arm/cpu_features.c
@@ -35,6 +35,7 @@
  * added to API level 18 for ARM and level 21 for AArch64.
  */
 
+#include "../cpu_features_common.h" /* must be included first */
 #include "cpu_features.h"
 
 #if ARM_CPU_FEATURES_ENABLED
@@ -91,6 +92,11 @@ out:
 	close(fd);
 }
 
+static const struct cpu_feature arm_cpu_feature_table[] = {
+	{ARM_CPU_FEATURE_NEON,		"neon"},
+	{ARM_CPU_FEATURE_PMULL,		"pmull"},
+};
+
 void setup_cpu_features(void)
 {
 	u32 features = 0;
@@ -113,6 +119,9 @@ void setup_cpu_features(void)
 		features |= ARM_CPU_FEATURE_PMULL;
 #endif
 
+	disable_cpu_features_for_testing(&features, arm_cpu_feature_table,
+					 ARRAY_LEN(arm_cpu_feature_table));
+
 	_cpu_features = features | ARM_CPU_FEATURES_KNOWN;
 }
 
diff --git a/lib/cpu_features_common.h b/lib/cpu_features_common.h
new file mode 100644
index 0000000..0c2cc6e
--- /dev/null
+++ b/lib/cpu_features_common.h
@@ -0,0 +1,83 @@
+/*
+ * cpu_features_common.h - code shared by all lib/$arch/cpu_features.c
+ *
+ * Copyright 2020 Eric Biggers
+ *
+ * Permission is hereby granted, free of charge, to any person
+ * obtaining a copy of this software and associated documentation
+ * files (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following
+ * conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+ * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ * OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#if defined(TEST_SUPPORT__DO_NOT_USE) && !defined(FREESTANDING)
+#  define _GNU_SOURCE 1 /* for strdup() and strtok_r() */
+#  include <stdio.h>
+#  include <stdlib.h>
+#  include <string.h>
+#endif
+
+#include "lib_common.h"
+
+struct cpu_feature {
+	u32 bit;
+	const char *name;
+};
+
+#if defined(TEST_SUPPORT__DO_NOT_USE) && !defined(FREESTANDING)
+/* Disable any features that are listed in $LIBDEFLATE_DISABLE_CPU_FEATURES. */
+static inline void
+disable_cpu_features_for_testing(u32 *features,
+				 const struct cpu_feature *feature_table,
+				 size_t feature_table_length)
+{
+	char *env_value, *strbuf, *p, *saveptr = NULL;
+	size_t i;
+
+	env_value = getenv("LIBDEFLATE_DISABLE_CPU_FEATURES");
+	if (!env_value)
+		return;
+	strbuf = strdup(env_value);
+	if (!strbuf)
+		abort();
+	p = strtok_r(strbuf, ",", &saveptr);
+	while (p) {
+		for (i = 0; i < feature_table_length; i++) {
+			if (strcmp(p, feature_table[i].name) == 0) {
+				*features &= ~feature_table[i].bit;
+				break;
+			}
+		}
+		if (i == feature_table_length) {
+			fprintf(stderr,
+				"unrecognized feature in LIBDEFLATE_DISABLE_CPU_FEATURES: \"%s\"\n",
+				p);
+			abort();
+		}
+		p = strtok_r(NULL, ",", &saveptr);
+	}
+	free(strbuf);
+}
+#else /* TEST_SUPPORT__DO_NOT_USE */
+static inline void
+disable_cpu_features_for_testing(u32 *features,
+				 const struct cpu_feature *feature_table,
+				 size_t feature_table_length)
+{
+}
+#endif /* !TEST_SUPPORT__DO_NOT_USE */
diff --git a/lib/x86/cpu_features.c b/lib/x86/cpu_features.c
index e931e62..e3471d4 100644
--- a/lib/x86/cpu_features.c
+++ b/lib/x86/cpu_features.c
@@ -25,6 +25,7 @@
  * OTHER DEALINGS IN THE SOFTWARE.
  */
 
+#include "../cpu_features_common.h" /* must be included first */
 #include "cpu_features.h"
 
 #if X86_CPU_FEATURES_ENABLED
@@ -75,6 +76,15 @@ read_xcr(u32 index)
 #define IS_SET(reg, nr)		((reg) & BIT(nr))
 #define IS_ALL_SET(reg, mask)	(((reg) & (mask)) == (mask))
 
+static const struct cpu_feature x86_cpu_feature_table[] = {
+	{X86_CPU_FEATURE_SSE2,		"sse2"},
+	{X86_CPU_FEATURE_PCLMUL,	"pclmul"},
+	{X86_CPU_FEATURE_AVX,		"avx"},
+	{X86_CPU_FEATURE_AVX2,		"avx2"},
+	{X86_CPU_FEATURE_BMI2,		"bmi2"},
+	{X86_CPU_FEATURE_AVX512BW,	"avx512bw"},
+};
+
 /* Initialize _cpu_features with bits for interesting processor features. */
 void setup_cpu_features(void)
 {
@@ -133,6 +143,9 @@ void setup_cpu_features(void)
 		features |= X86_CPU_FEATURE_AVX512BW;
 
 out:
+	disable_cpu_features_for_testing(&features, x86_cpu_feature_table,
+					 ARRAY_LEN(x86_cpu_feature_table));
+
 	_cpu_features = features | X86_CPU_FEATURES_KNOWN;
 }