The cytotoxic activities of the peripheral lymphocytes prepared from two retinoblastoma patients to freshly isolated autologous tumor cells were measured using 51Cr-release assay. The retinoblastoma cells of the two patients resisted lysis by unstimulated effectors, whereas interferon(IFN)-stimulated effector cells gave a significantly higher cytotoxicity. This result implies that IFN therapy may be potent in treating high-risk retinoblastoma patients.