C57BL/6N mice immunized with regressor murine sarcoma virus (MuSV) were studied at different times after inoculation for their cellular immune responses as measured by migration inhibition assays. We used both the direct capillary and indirect agarose dropiet methods and, as the source of antigens, viable tumor cells and 3 M KCl-solubilized extracts. Migration inhibitory factor (MIF) production coulctivity becoming undetectable after 21 days. However, the level of activity and the kinetics of production of this lymphokine were strongly influenced by the source of the antigen and the form in which it was presented to the immune spleen cells (ISC). Using RBL-5 ct 9-10 days, a decrease to low levels at 14 days, and a second peak of activity between 17 and 21 days. However, uith MBL-2 cells or with 3 M KCl-solubilized antigen from fresh RBL-5 ascites cells, MIF production was observed as early as 9-10 days after tumor induction, peaked at 14 days, and decreased substantially by 21 days. T-cells appeared to be required for migration inhibition reactivity, since ISC depleted of T-lymphocytes by treatment with antibody plus complement were unable to produce MIF after antigen stimulation. The results obtained from specificity studies on the response of ISC in migration inhibition to 11 different tumor lines agreed with the results previously obtained from cytotoxicity studies. With the use of RBL-5 cells as the antigen, there appeared to be an inverse relationship between the development of specific cytotoxic effector cells in 51Cr-release assay and the development of specific effector cells needed for MIF production. However, after removal of adherent cells, the level of cytotoxicity observed correlated with MIF release by immune lymphocytes.