Abstract
The immune response to progressor murine sarcoma virus (P‐MuSV)^2^ and regressor murine sarcoma virus (R‐MuSV) was analyzed by the radioisotopic footpad assay (FPA) for delayed hypersensitivity (DH) and by the membrane immunofluorescence assay for anti‐tumor antibodies. In mice injected with P‐MuSV, the footpad response remained at a low level for 7 weeks, while membrane immunofluorescence antibody titers reached their maximum levels after 5 weeks. By contrast, in mice injected with R‐MuSV, the footpad response rose to a maximum by 4 weeks and membrane immunofluorescence antibody titers reached a maximum after 4 weeks. The adoptive footpad response reached its highest level in 2 weeks. A most striking difference between the P‐MuSV and the R‐MuSV systems was the lower immunogenicity of tumor cells induced by the P‐MuSV as measured by the FPA. Considerable differences between the two stocks were also observed in oncogenic activity and tumor growth patterns. The P‐MuSV contained one‐tenth the amount of helper Moloney leukemia virus found in R‐MuSV. Addition of more helper Moloney leukemia virus to the P‐MuSV did not produce regressor tumors.