diff --git a/src/H5FDs3comms.c b/src/H5FDs3comms.c
index f5be8f42f97..bcda801a8dd 100644
--- a/src/H5FDs3comms.c
+++ b/src/H5FDs3comms.c
@@ -244,7 +244,7 @@ H5FD_s3comms_hrb_node_set(hrb_node_t **L, const char *name, const char *value)
     if (lowername == NULL)
         HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "cannot make space for lowercase name copy.");
     for (i = 0; i < namelen; i++)
-        lowername[i] = (char)HDtolower((int)name[i]);
+        lowername[i] = (char)tolower((int)name[i]);
     lowername[namelen] = 0;
 
     /* If value supplied, copy name, value, and concatenated "name: value".
@@ -2172,7 +2172,7 @@ H5FD_s3comms_nlowercase(char *dest, const char *s, size_t len)
         H5MM_memcpy(dest, s, len);
         do {
             len--;
-            dest[len] = (char)HDtolower((int)dest[len]);
+            dest[len] = (char)tolower((int)dest[len]);
         } while (len > 0);
     }
 
@@ -2264,7 +2264,7 @@ H5FD_s3comms_parse_url(const char *str, parsed_url_t **_purl)
     strncpy(purl->scheme, curstr, (size_t)len);
     purl->scheme[len] = '\0';
     for (i = 0; i < len; i++)
-        purl->scheme[i] = (char)HDtolower(purl->scheme[i]);
+        purl->scheme[i] = (char)tolower(purl->scheme[i]);
 
     /* Skip "://" */
     tmpstr += 3;
diff --git a/src/H5private.h b/src/H5private.h
index e0148f43d70..b8d65405122 100644
--- a/src/H5private.h
+++ b/src/H5private.h
@@ -893,12 +893,6 @@ H5_DLL H5_ATTR_CONST int Nflock(int fd, int operation);
 #ifndef HDtmpfile
 #define HDtmpfile() tmpfile()
 #endif
-#ifndef HDtolower
-#define HDtolower(C) tolower(C)
-#endif
-#ifndef HDtoupper
-#define HDtoupper(C) toupper(C)
-#endif
 #ifndef HDunlink
 #define HDunlink(S) unlink(S)
 #endif
diff --git a/src/H5system.c b/src/H5system.c
index 1379fa7dc95..65b56fd8d81 100644
--- a/src/H5system.c
+++ b/src/H5system.c
@@ -689,7 +689,7 @@ H5_build_extpath(const char *name, char **extpath /*out*/)
          * Unix: does not apply
          */
         if (H5_CHECK_ABS_DRIVE(name)) {
-            drive  = HDtoupper(name[0]) - 'A' + 1;
+            drive  = toupper(name[0]) - 'A' + 1;
             retcwd = HDgetdcwd(drive, cwdpath, MAX_PATH_LEN);
             strncpy(new_name, &name[2], name_len);
         }
@@ -1404,7 +1404,7 @@ H5_strcasestr(const char *haystack, const char *needle)
         const char *h = haystack;
         const char *n = needle;
         /* loop while lowercase strings match, or needle ends */
-        while (HDtolower(*h) == HDtolower(*n) && *n) {
+        while (tolower(*h) == tolower(*n) && *n) {
             h++;
             n++;
         }