Mutagen treatment of mouse mastocytoma P815 produces highly immunogenic "turn-" variants. Most of these variants express potent transplantation antigens which are not present on the original P815 tumor cells. These tum-antigens, which appear to be specific for each variant, elicit a strong cytolytic T lymphocyte (CTL) response, but do not seem to induce a specific antibody response. As a first step in the isolation of the gene of a turn-antigen, we attempted DNA-mediated gene transfer. As a DNA recipient cell we used P1.HTR, a highly transfectable P815 cell line, whose selection has been previously described. For the detection of antigenexpressing cells in transfected populations we developed a procedure that relies on the ability of these cells to stimulate the proliferation of the relevant CTL. Using DNA from turn-variant P91 mixed with a plasmid carrying an antibiotic resistance gene, we obtained several independent transfectants expressing a turn-antigen, at a frequency of approximately 1 in 13 000 antibiotic-resistant transfectants. These transfectants express only one of the two turn-antigens that were identified on P91, suggesting that these tum-antigens correspond to different genes. We expect that the detection procedure described here will be-suitable for the identification of transfectants for any gene that determines the expression of an antigen recognized by CTL.