To obtain a rationale for the use of radiation in the treatment of proliferative vitreoretinopathy (PVR) and to determine the appropriate dose of irradiation, we studied the in vitro growth of cultured human retinal pigment epithelial (RPE) cells after various doses of irradiation. At 48 h after plating, doses of 100-3000 cGy were given to the cells by Linac X-ray. On days 1, 3, and 7 after irradiation, cell growth was evaluated by cell number and DNA synthesis. At 7 days after irradiation, the cell number declined to 42% and 3.7% of control values after doses of 300 and 1000 cGy, respectively. Almost the same inhibitory effects were observed following doses of 1000 cGy and 3000 cGy. DNA synthesis was suppressed to 67.6% and 2.8% of control values on the 7th day after radiation doses of 500 and 1000 cGy, respectively. Low-dose irradiation can be used as adjunctive therapy for PVR, and doses of 500-1000 cGy are most likely needed.