SUMMARY The in vitro migration of peritoneal macrophages from strain 13 guinea pigs immunized with tumor tissue from strain 2 guinea pigs was significantly inhibited when exposed to strain 2 tumor or normal cells. No inhibition was observed when macrophages from normal strain 13 guinea pigs were used or when macrophages from immunized strain 13 guinea pigs were exposed to strain 13 cells. These observations suggest that this technique may offer a convenient in vitro system for study of the cellular aspects of homograft immunity