Human peripheral blood mononuclear cells were used as effectors against the Wehi 164 mouse fibrosarcoma cell line grown in suspension culture. In a standard 7-hour 51Cr release assay, specific release usually was below 10%. In contrast, pretreatment of Wehi 164 for 3 hours with dactinomycin (Act D), while leaving the tumor cells intact and viable, resulted in a drastic increase in its susceptibility to lysis, which reached 60% specific release. In terms of lytic units, this reflects up to a fiftyfold enhancement. Cell-separation experiments revealed that the effector cells were plastic-adherent and iron-phagocytic. Adherent cell fractions with 85-98% naphthol AS acetate-esterase (NAS)-positive cells were enriched in cytotoxicity, while nonadherent cells with less than 4% NAS-positive cells were almost devoid of activity. Depletion of phagocytic cells with iron and magnet resulted in a strong reduction of cytotoxicity by 82-95% compared to the cytotoxicity seen in control treated effector cells. In both kinds of analyses, natural killer cell activity showed a reciprocal behavior. The evidence indicates that the cytotoxic effector cells directed against Act D-treated Wehi 164 cells belong to the monocyte lineage. The system described should be useful in analyzing the cytotoxic function of unstimulated monocytes in a short-term assay without prior purification of the effector cells.