diff --git a/src/cmd/5a/a.h b/src/cmd/5a/a.h
index 37791f85052..86dbac26b6a 100644
--- a/src/cmd/5a/a.h
+++ b/src/cmd/5a/a.h
@@ -39,7 +39,7 @@
 
 #define	getc	aagetc
 #define	ungetc	aaungetc
-#define	BUFSIZ	AABUFSIZ
+#undef	BUFSIZ
 
 typedef	struct	Sym	Sym;
 typedef	struct	Gen	Gen;
diff --git a/src/cmd/6a/a.h b/src/cmd/6a/a.h
index 98dcc17d13c..ce2ab292274 100644
--- a/src/cmd/6a/a.h
+++ b/src/cmd/6a/a.h
@@ -40,7 +40,7 @@
 
 #define	getc	aagetc
 #define	ungetc	aaungetc
-#define	BUFSIZ	AABUFSIZ
+#undef	BUFSIZ
 
 typedef	struct	Sym	Sym;
 typedef	struct	Ref	Ref;
diff --git a/src/cmd/8a/a.h b/src/cmd/8a/a.h
index 70682e6b564..37b175c1891 100644
--- a/src/cmd/8a/a.h
+++ b/src/cmd/8a/a.h
@@ -40,7 +40,7 @@
 
 #define	getc	aagetc
 #define	ungetc	aaungetc
-#define	BUFSIZ	AABUFSIZ
+#undef	BUFSIZ
 
 typedef	struct	Sym	Sym;
 typedef	struct	Ref	Ref;
diff --git a/src/cmd/cc/cc.h b/src/cmd/cc/cc.h
index 51944d92a30..696cdab5671 100644
--- a/src/cmd/cc/cc.h
+++ b/src/cmd/cc/cc.h
@@ -41,7 +41,7 @@
 
 #define	getc	ccgetc
 #define	ungetc	ccungetc
-#define	BUFSIZ	GCBUFSIZ
+#undef	BUFSIZ
 
 typedef	struct	Node	Node;
 typedef	struct	Sym	Sym;
diff --git a/src/cmd/gc/go.h b/src/cmd/gc/go.h
index d92075df6d8..33f93b7d772 100644
--- a/src/cmd/gc/go.h
+++ b/src/cmd/gc/go.h
@@ -17,7 +17,7 @@
 
 #define	getc	gcgetc
 #define	ungetc	gcungetc
-#define	BUFSIZ	GCBUFSIZ
+#undef	BUFSIZ
 
 enum
 {