Retinoblastoma (Rb) cells may be more sensitive to X-irradiation than normal cells when assayed for clonogenic survival, suggesting that such sensitivity can be used as a trait for detecting RB1 gene carriers. We studied induction of chromosome damage in lymphocytes from patients with Rb by the radiomimetic antineoplastic antibiotic, bleomycin (BL). Lymphocytes were treated in G2 for 4 h at four concentrations of the drug (0.0, 0.3, 0.9, and 2.7 micrograms/ml), and chromosome damage was evaluated. No difference in levels of spontaneous or induced damage was detected among hereditary Rb, non-hereditary Rb, and normal lymphocytes. The role of X-ray treatment in the induction of second tumors in Rb patients is discussed.