The immunogenicity of cyclophosphamide-treated murine tumor cells was tested on syngeneic or semisyngeneic mice. The majority of mice immunized with modified Ehrlich carcinoma cells, L1210 leukemia cells or AA sarcoma cells survived after challenge with a lethal dose of unmodified tumor cells. Plasmacytomas RPC-5 and MOPC 104E were less immunogenic and L-1 sarcoma cells were not immunogenic.