In tumor-bearing rats inoculated intramuscularly with 5 x 10(4) SLC cells, the cytostatic activity of peritoneal macrophages was elevated in the early stage (7 days after transplantation) and decreased in the advanced stage (21 days after transplantation). When BCG was intraperitoneally administered into normal and tumor-bearing rats, peritoneal macrophages showed higher cytostatic activity than the untreated macrophages. This elevated cytostatic activity of the marcophages obtained from BCG-treated tumor-bearing rats was maintained even at an advanced stage.