We have reported that it is possible to obtain variants that are incapable of forming progressive tumour in syngeneic mice (tum-) by mutagenesis and cloning of a teratocarcinoma and a Lewis lung carcinoma cell line. These observations were extended to the ascitic P815 mastocytoma of mouse strain DB/2. After a treatment with the mutagen N-methyl-N"-nitro-N-nitrosoguanidine, we obtained a high frequency (14%) of tum- clones. A colony assay indicated that after a period of rapid multiplication extending to approximately 10 d after injection, the P815 tum- cells were rejected by a process that was usually completed by day 15. No rejection was observed in sublethally irradiated animals. The immunological nature of the rejection of the P815 variants was further inferred because, upon rejection, the mice acquired a radioresistant specific protection that could be transferred adoptively with spleen cells. Cross-immunization patterns demonstrated the presence of singular antigenic specificities on three of the five variants that were examined. In addition, a common antigen was found on all the tum- variants and the original cells that were capable of forming progressive tumors in syngeneic mice (tum+). Mice injected with tum- cells were significantly protected against a tum+ challenge, even though no significant protection was generated by irradiated tum+ cells. A study of the T lymphocyte-mediated cytolysis against the P815 variants described here is presented in the accompanying report (9).