Mononuclear phagocytes were isolated from the peripheral blood (PB) and ascites tumors of 35 patients with epithelial ovarian tumors. After 48 hours of incubation with the TU5 tumor, tumor-associated macrophages (TAM) and PB monocytes from cancer patients showed lower cytolytic activity than did control cells, but by 72 hours there was little difference between control and ovarian cancer effector cells. Primary ovarian carcinoma cultures were heterogeneous in their susceptibility to macrophage cytotoxicity. Tumor cells from 7 patients were significantly lysed by monocytes and macrophages, whereas four ovarian cancer cell preparations were resistant to cytotoxicity. A "feeding" effect of mononuclear phagocytes on non-lysable tumor cells was detected in terms of both lower [3H]thymidine-release values in the cytolysis assay and increased proliferation in cytostasis assays. Thus patients with ovarian carcinomatous ascites PB monocytes and TAM had impaired cytotoxicity against a tumor cell line, and primary ovarian carcinoma cultures were heterogeneous in their interaction with mononuclear phagocytes.