In three unrelated families with transmitted retinoblastoma 43 members were tested for HLA-A, B, C, and DR antigens. On 18 patients, 16 unaffected relatives, and 16 controls, mixed lymphocyte cultures were carried out and the response to mitogens, phytohaemagglutinin, concanavalin A, and pokeweed mitogen examined. There were no associations of retinoblastoma with any HLA antigen. A reduction in T cell function is suggested by the results of the mitogen tests.