diff --git a/libguile/smob.c b/libguile/smob.c
index 8b8091c04..e7e75cf14 100644
--- a/libguile/smob.c
+++ b/libguile/smob.c
@@ -164,13 +164,14 @@ scm_smob_trampoline (unsigned int nreq, unsigned int nopt,
 
 
 scm_t_bits 
-scm_make_smob_type (char const *name, size_t size)
-#define FUNC_NAME "scm_make_smob_type"
+scm_make_managed_smob_type (char const *name, size_t nfields)
+#define FUNC_NAME "scm_make_smob_type_with_gc_info"
 {
-  long new_smob;
+  if (nfields > 32)
+    scm_misc_error (FUNC_NAME, "maximum 32 managed fields", SCM_EOL);
 
   scm_i_pthread_mutex_lock (&scm_i_misc_mutex);
-  new_smob = scm_numsmob;
+  long new_smob = scm_numsmob;
   if (scm_numsmob != MAX_SMOB_COUNT)
     ++scm_numsmob;
   scm_i_pthread_mutex_unlock (&scm_i_misc_mutex);
@@ -179,7 +180,7 @@ scm_make_smob_type (char const *name, size_t size)
     scm_misc_error (FUNC_NAME, "maximum number of smobs exceeded", SCM_EOL);
 
   scm_smobs[new_smob].name = name;
-  scm_smobs[new_smob].size = size;
+  scm_smobs[new_smob].field_count = nfields;
 
   /* Make a class object if Goops is present. */
   if (SCM_UNPACK (scm_i_smob_class[0]) != 0)
@@ -189,6 +190,22 @@ scm_make_smob_type (char const *name, size_t size)
 }
 #undef FUNC_NAME
 
+scm_t_bits
+scm_make_smob_type (char const *name, size_t size)
+{
+  scm_t_bits tc = scm_make_managed_smob_type (name, 0);
+  scm_smobs[SCM_TC2SMOBNUM (tc)].size = size;
+  return tc;
+}
+
+void
+scm_set_smob_field_is_unmanaged (scm_t_bits tc, size_t idx)
+{
+  struct scm_smob_descriptor *desc = &scm_smobs[SCM_TC2SMOBNUM (tc)];
+  if (idx >= desc->field_count)
+    abort ();
+  scm_smobs[SCM_TC2SMOBNUM (tc)].unmanaged_fields |= 1U << idx;
+}
 
 void
 scm_set_smob_free (scm_t_bits tc, size_t (*free) (SCM))
diff --git a/libguile/smob.h b/libguile/smob.h
index 22d925b8b..091f5c66a 100644
--- a/libguile/smob.h
+++ b/libguile/smob.h
@@ -40,6 +40,8 @@ typedef struct scm_smob_descriptor
   SCM (*equalp) (SCM, SCM);
   scm_t_subr apply;
   SCM apply_trampoline;
+  size_t field_count;
+  uint32_t unmanaged_fields;
 } scm_smob_descriptor;
 
 
@@ -193,6 +195,10 @@ SCM_API int scm_smob_print (SCM exp, SCM port, scm_print_state *pstate);
  */
 
 SCM_API scm_t_bits scm_make_smob_type (char const *name, size_t size);
+SCM_API scm_t_bits scm_make_managed_smob_type (char const *name,
+                                               size_t nfields);
+
+SCM_API void scm_set_smob_field_is_unmanaged (scm_t_bits tc, size_t idx);
 
 SCM_API void scm_set_smob_free (scm_t_bits tc, size_t (*free) (SCM));
 SCM_API void scm_set_smob_print (scm_t_bits tc,