diff --git a/include/extern.h b/include/extern.h
index c4e33ba70..14a48f6a1 100644
--- a/include/extern.h
+++ b/include/extern.h
@@ -1694,7 +1694,9 @@ extern const char *msummon_environ(struct permonst *, const char **);
 extern const struct permonst *raceptr(struct monst *);
 extern boolean olfaction(struct permonst *);
 unsigned long cvt_adtyp_to_mseenres(uchar);
+unsigned long cvt_prop_to_mseenres(uchar);
 extern void monstseesu(unsigned long);
+extern void monstunseesu(unsigned long);
 extern boolean resist_conflict(struct monst *);
 extern boolean mon_knows_traps(struct monst *, int);
 extern void mon_learns_traps(struct monst *, int);
diff --git a/include/monst.h b/include/monst.h
index 8bed8cf6e..2f4800283 100644
--- a/include/monst.h
+++ b/include/monst.h
@@ -88,7 +88,9 @@ enum m_seen_resistance {
 
 #define m_seenres(mon, mask) ((mon)->seen_resistance & (mask))
 #define m_setseenres(mon, mask) ((mon)->seen_resistance |= (mask))
+#define m_clearseenres(mon, mask) ((mon)->seen_resistance &= ~(mask))
 #define monstseesu_ad(adtyp) monstseesu(cvt_adtyp_to_mseenres(adtyp))
+#define monstunseesu_prop(prop) monstunseesu(cvt_prop_to_mseenres(prop))
 
 struct monst {
     struct monst *nmon;
diff --git a/src/mondata.c b/src/mondata.c
index 312920c75..282da195a 100644
--- a/src/mondata.c
+++ b/src/mondata.c
@@ -1450,7 +1450,25 @@ cvt_adtyp_to_mseenres(uchar adtyp)
     }
 }
 
-/* Monsters remember hero resisting effect M_SEEN_foo */
+/* Convert property resistance to M_SEEN_bar */
+unsigned long
+cvt_prop_to_mseenres(uchar prop)
+{
+    switch (prop) {
+    case ANTIMAGIC: return M_SEEN_MAGR;
+    case FIRE_RES: return M_SEEN_FIRE;
+    case COLD_RES: return M_SEEN_COLD;
+    case SLEEP_RES: return M_SEEN_SLEEP;
+    case DISINT_RES: return M_SEEN_DISINT;
+    case POISON_RES: return M_SEEN_POISON;
+    case SHOCK_RES: return M_SEEN_ELEC;
+    case ACID_RES: return M_SEEN_ACID;
+    case REFLECTING: return M_SEEN_REFL;
+    default: return M_SEEN_NOTHING;
+    }
+}
+
+/* Monsters in line of sight remember hero resisting effect M_SEEN_foo */
 void
 monstseesu(unsigned long seenres)
 {
@@ -1464,6 +1482,20 @@ monstseesu(unsigned long seenres)
             m_setseenres(mtmp, seenres);
 }
 
+/* Monsters in line of sight forget hero resistance to M_SEEN_foo */
+void
+monstunseesu(unsigned long seenres)
+{
+    struct monst *mtmp;
+
+    if (seenres == M_SEEN_NOTHING || u.uswallow)
+        return;
+
+    for (mtmp = fmon; mtmp; mtmp = mtmp->nmon)
+        if (!DEADMONSTER(mtmp) && m_canseeu(mtmp))
+            m_clearseenres(mtmp, seenres);
+}
+
 /* Can monster resist conflict caused by hero?
 
    High-CHA heroes will be able to 'convince' monsters
diff --git a/src/worn.c b/src/worn.c
index f09ea5374..96ba2ddd2 100644
--- a/src/worn.c
+++ b/src/worn.c
@@ -71,6 +71,10 @@ setworn(struct obj *obj, long mask)
                         p = objects[oobj->otyp].oc_oprop;
                         u.uprops[p].extrinsic =
                             u.uprops[p].extrinsic & ~wp->w_mask;
+                        /* if the hero removed an extrinsic-granting item,
+                           nearby monsters will notice and attempt attacks of
+                           that type again */
+                        monstunseesu_prop(p);
                         if ((p = w_blocks(oobj, mask)) != 0)
                             u.uprops[p].blocked &= ~wp->w_mask;
                         if (oobj->oartifact)
@@ -132,6 +136,7 @@ setnotworn(struct obj *obj)
             *(wp->w_obj) = (struct obj *) 0;
             p = objects[obj->otyp].oc_oprop;
             u.uprops[p].extrinsic = u.uprops[p].extrinsic & ~wp->w_mask;
+            monstunseesu_prop(p); /* remove this extrinsic from seenres */ 
             obj->owornmask &= ~wp->w_mask;
             if (obj->oartifact)
                 set_artifact_intrinsic(obj, 0, wp->w_mask);