A7-NCS, which is a conjugate of the monoclonal antibody A7 against a human colonic cancer and the anticancer agent neocarzinostatin (NCS), reacts with human gastric cancers at a high rate. The anticancer effect of A7-NCS is stronger than that of free NCS. Nude mice models of peritoneal dissemination were established simply by intra-peritoneal inoculation of the antigen-positive human gastric cancer cell line MKN45. Using these models, the anticancer effect of A7-NCS against the peritoneal dissemination of gastric cancer was examined. The murine peritoneal dissemination models were divided into three groups. The anticancer effect of the group injected intra-peritoneally with A7-NCS 2 days after cancer inoculation was compared with that injected 18 days after inoculation. The anticancer effect in the 18-day group was inferior to that in the 2-day group, but was superior to that in the non-treated group. To get a better result in the 18-day group, which is in the advanced stage of peritoneal dissemination, repeated injection and the dose of A7-NCS should be examined.