The direct tumor inhibiting capacity of spleen and lymph node cells obtained from mice in different stages of Moloney sarcoma virus (MSV-M)-induced tumor development was studied. MSV-M producing ascitic tumor cells with a tumor dosis50 of about 10(3) cells and a mean latent period--depending on the amount of transplanted cells--of6-12 days were used as target cells. They were mixed with dilutions of spleen and lymph node cells obtained from mice at various times after infection with MSV-M. The cell mixtures were incubated for 5 hours, then injected into mice, and tumor inhibiting activity was determined after an observation period of 14 days. Tumor inhibiting capacity was demonstrated with cells harvested from mice 8 days after infection with MSV-M, it was strongest with cells obtained at the stage of peak tumor size and during regression, and was still demonstrable with cells taken at the 86th day after infection. Pretreatment of the lymphoid cells with anti-0-antibodies and complement abolished the tumor inhibiting capacity. This result indicates that immune T cells abrogate the capacity of the ascitic tumor cells to induce tumor development.