To test the role of antitumor CD4 T effector cells as part of a natural endogenous response, we depleted female C57Bl/6 (B6) mice of their CD4 or CD8 T cells 5 days before challenging them with the bladder carcinoma MB49 cell line, which spontaneously expresses the H-Y male antigen. This evidence concerns the gene CD4 and urinary bladder carcinoma.