Peritoneal macrophages obtained from normal CBA mice expressed significant cytotoxicity against the DBA/2-derived P815 mastocytoma but not against DBA/2-derived SL2 tumor or the C57BL-derived tumors TLX9 and EL4. The macrophages also expressed some cytotoxicity against the DBA/2-derived L5178Y tumor. Incubation of normal CBA macrophages with cell-free exudate of intraperitoneally growing P815 cells resulted in cytotoxicity against the SL2 and against the EL4 lymphosarcoma. Incubation of SL2 tumor cells with P815 ascitic fluid before adding the SL2 tumor cells to normal CBA macrophage monolayers also resulted in inhibition of SL2 tumor cell growth on these monolayers. Trypsinization of the macrophages after incubation with ascitic fluid (or tumor extract) but before challenge with tumor cells abolished cytotoxic activity of the macrophages. Incubation of normal macrophages with a comparable amount of trypsin before tumor cells were added had no influence on their activity. Cytotoxicity could be induced after 7 days storage of the exudate at 5 degrees C, but this ability was lost within 72 hr when kept at room temperature. Storage at -20 degrees C had no influence. Gel fractionation of the cell-free exudate showed that the product responsible for the effect is a small molecular weight product (mol wt less than 1000). Furthermore the "product" was dialyzable. The "factor" could not be shown in the supernatant of P815 cell cultures unless the cultures comprised greater than or equal to 40% dead cells.