Mutagen treatment of mouse P815 tumor cells produces immunogenic mutants that express new transplantation antigens (turn-antigens) recognized by cytolytic T cells. We found that the gene conferring expression of turn-antigen PQlA contains 12 exons, encoding a 60 kd protein lacking a typical N-terminal signal sequence. The sequence shows no significant similarity with sequences in current data bases. A mutation that causes expression of the antigen is located in exon 4; it is the only apparent difference between the normal and the antigenic alleles. A short synthetic peptide corresponding to a region of exon 4 located around this mutation makes P815 cells sensitive to lysis by anti-PQlA cytolytic T cells. The mutation creates a strong aggretope enabling the peptide to bind the H-2 Ld molecule. Several secondary tumor cell variants that no longer express turn-antigen PQlA were found to carry deletions in the gene.