The phenotypes of the affector and effector cell populations involved during the induction of permanent tumor regression by combined therapy using cyclophosphamide (CY) and immune spleen cells were identified. Permanent tumor regression was dependent on the presence of Thyl+ Lyt-2+ lymphocytes in the immune spleen cell population that was injected i.v. 2-4 h after an intraperitoneal injection of CY (240 mg/kg) into C57BL/6J mice bearing 1.0-1.5 g. MCA/76-9 or MCA/76-64 sarcomas. Histological evaluation after the combined treatment indicated an intense influx of putative lymphocytes 6-10 days after the combined treatment, over and above the inflammatory response involving macrophages and neutrophils induced by CY treatment alone. These infiltrating cells were shown to be T lymphocytes, most of them having the Lyt-1 and Lyt-2 antigens on their cell membrane. Isolated MCA/76-9 or 76-64 tumor-associated cells (TAC), lymphocytes (TAL) and macrophages (TAM) inhibited MCA/76-9 or 76-64 tumor growth respectively in a Winn test in an immunologically specific manner, having no effect on the growth of the B6 sarcoma cells, MCA/76-45 and 77-23. The tumor-free mice from the Winn test were not resistant to a subsequent challenge inoculum of either MCA/76-9 or 76-64 cells. Isolated TAC were usually non-specifically cytotoxic in vitro, while TAL and TAM showed some degree of specificity. The overall data indicated that the ultimate rejection of those tumor cells remaining after the direct anti-tumor action of CY had been dissipated was probably mediated by the combined action of TAL and TAM.