diff --git a/biz/models.py b/biz/models.py
index f7da7bc..ffc6103 100644
--- a/biz/models.py
+++ b/biz/models.py
@@ -144,6 +144,11 @@ class Activity(BaseModel):
     def delete_dependencies(self):
         pass
 
+    def save(self, *args, **kwargs):
+        super().save(*args, **kwargs)
+        # Update last_update for all related prospects when activity is saved
+        self.prospects.update(last_update=timezone.now())
+
     class Meta:
         verbose_name_plural = "Activities"
         ordering = ['-creation_date']