diff --git a/src/liblzma/api/lzma/container.h b/src/liblzma/api/lzma/container.h
index e68c8940..499d8b9a 100644
--- a/src/liblzma/api/lzma/container.h
+++ b/src/liblzma/api/lzma/container.h
@@ -60,6 +60,7 @@
 #define LZMA_PRESET_EXTREME       (UINT32_C(1) << 31)
 
 
+#ifdef LZMA_UNSTABLE /* Unstable API that may change. Use only for testing. */
 /**
  * \brief       Multithreading options
  */
@@ -179,6 +180,7 @@ typedef struct {
 	void *reserved_ptr4;
 
 } lzma_mt;
+#endif
 
 
 /**
@@ -311,6 +313,7 @@ extern LZMA_API(lzma_ret) lzma_stream_encoder(lzma_stream *strm,
 		lzma_nothrow lzma_attr_warn_unused_result;
 
 
+#ifdef LZMA_UNSTABLE /* Unstable API that may change. Use only for testing. */
 /**
  * \brief       Calculate approximate memory usage of multithreaded .xz encoder
  *
@@ -351,6 +354,7 @@ extern LZMA_API(uint64_t) lzma_stream_encoder_mt_memusage(
 extern LZMA_API(lzma_ret) lzma_stream_encoder_mt(
 		lzma_stream *strm, const lzma_mt *options)
 		lzma_nothrow lzma_attr_warn_unused_result;
+#endif
 
 
 /**
diff --git a/src/liblzma/common/common.h b/src/liblzma/common/common.h
index 8e9a387c..9d776f9a 100644
--- a/src/liblzma/common/common.h
+++ b/src/liblzma/common/common.h
@@ -32,6 +32,8 @@
 
 #define LZMA_API(type) LZMA_API_EXPORT type LZMA_API_CALL
 
+#define LZMA_UNSTABLE
+
 #include "lzma.h"
 
 // These allow helping the compiler in some often-executed branches, whose
diff --git a/src/xz/private.h b/src/xz/private.h
index 6b01e513..978f81a3 100644
--- a/src/xz/private.h
+++ b/src/xz/private.h
@@ -12,6 +12,8 @@
 
 #include "sysdefs.h"
 #include "mythread.h"
+
+#define LZMA_UNSTABLE
 #include "lzma.h"
 
 #include <sys/types.h>