Abstract Spleen cells from female mice bearing progressively-growing MTV-induced mammary tumors reduced tumor cell survival in microcytotoxicity tests. Serum from these females contained blocking factors which inhibited spleen cell activity either partially or completely, depending upon the method of serum assay, whenever serum and target cells had not been obtained from the same donor. Serum obtained from target cell donors had a reverse effect when tested by two of three serum assay methods; tumor cell survival was significantly decreased. More than one type of mammary tumor cell antigen (and host response) was detected in these tests.