&NA;Abdominal operations induce immunosuppression during the time when tumors are manipulated and tumor cells are released into the circulation. The authors tested the hypothesis that the combined effect of these factors may promote the development of metastatic tumor implants and that perioperative treatment with Human Recombinant Interleukin‐2 (RIL‐2), a known immunostimulant oft, natural killer (NK), and lymphokine activated killer (LAK) cells may reduce the incidence of liver metastases from transplantable rat colon cancers. Hepatic metastases were induced in male Fischer 344 (F344) rats by injecting 107rat colon tumor cells into the portal venous system during laparotomy. Control rats developed tumors by four weeks and were dead by ten weeks. Eleven groups of rats underwent celiotomy with portal vein injection of tumor on day three. Rats received either no RIL‐2, RIL‐2, or excipient buffer at varying doses on days 1 through 5 or 3 through 7 of these experiments. Animals were assessed for the presence of tumor and the incidence of liver metastases at autopsy (sacrifice and autopsy performed at seven weeks). Eighty‐five percent of the rats in the untreated group developed tumor. This compared with only 50 percent of animals receiving 103u/dose (P<.025) and 42 percent of animals receiving 104u/dose (P<.01) of Interleukin‐2 on days 1 through 5. Animals receiving very high doses of RIL‐2 (105or 4×105units per dose) had a greater chance of developing tumors than did control rats, or rats receiving lower doses of RIL‐2 (P<.05). It is concluded that the perioperative period may be critical for the implantation and growth of metastatic disease and that perioperative immunostimulation with RIL‐2 can decrease the incidence of tumors in these animals. This model may have relevance to the treatment of human colon cancer.