Quantitative studies using a 51Cr release assay were performed to analyse the time-course of appearance and specificity of cytolytic cells in C57Bl/6 mice inoculated with Moloney murine sarcoma virus (MSV). Various lymphoid organs were studied including spleen, mesenteric lymph nodes (MLN), lymph nodes regional to the MSV-induced tumor (RLN), and peripheral blood. Furthermore, the kinetics of appearance of cytolytic cells within the MSV-induced tumor was determined. In agreement with previous studied, it was found that cytolytic T lymphocytes (CTL) specific for MSV-associated antigens were present among the cells extracted from the tumor and from the lymphoid organs. However, differences in the kinetics of CTL activity could be documented: lymphoid cells from spleen, RLN and peripheral blood showed peak activity at the time of maximum tumor diameter, while intratumoral cells showed peak activity at the onset of tumor regression. MLN cells showed cytolytic activity only when tumor regression had begun. Naturally cytotoxic cell populations of thymus-independent origin, present in normal control mice, were also detected in MSV-infected mice.