Prostaglandin D2 (PGD2) and 64E, a derivative of PGs, were tested for their capacity to suppress the growth of retinoblastoma in the nude mouse. Ten million cells of an established cell line of human retinoblastoma, Y-79 cells, were transferred subcutaneously into the nude mouse, and after the transferred cells became a tumor with a diameter larger than 7.5 mm, either PGD2 (1, 2, or 4 mg/kg/day) or 64E (4 mg/kg/day) dissolved in Hanks' solution was daily injected subcutaneously near the tumor for 14 days. The estimated tumor weight as defined by the formula of (length) X (width)2/2 was evaluated at various time intervals after the treatment. Although a low dose of PGD2, 1 mg/kg/day, had no effect, higher doses of PGD2 had a clear effect in suppressing the growth of retinoblastoma in the nude mouse. Tumors in 64E-treated animals were also markedly suppressed in their growth. Histological examination revealed that tumors treated with these drugs had a much larger area of necrosis with fewer tumor cells than the tumors in control animals.