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It has been reported that retinoic acid (RA) may inhibit the growth of RPE and be used in the treatment of proliferative vitreoretinopathy (PVR). However, previous reports in this field have been conflicting. The main reason for these contradictory findings is that different methods for evaluating the effects of RA on RPE from different species have been used. In human specimens, only RPE from the donor eye (stationary) but not RPE from the PVR membrane (already at active proliferation status) have been tested. This study tested the effects of RA on the growth of RPE using a novel in vitro model: RPE from the PVR membranes, which simulates the in vivo situation of PVR patients better than RPE from the donor eyes. This study also used various methods to solve the conflicting results reported previously. We found that both all transretinoic acid (all-RA) and 13-cis-retinoic acid (cis-RA) can promptly (though not completely) inhibit proliferation of RPE (inhibition rate of 89%-90%) over a very wide range of concentrations (10-9-10-5 M) and various lengths of periods (2-12 days) in a dosedependent and time-dependent manner and without evident cytotoxic effects. Previously reported disadvantages discovered from the study of RPE from donor eyes, e.g., the absence of inhibitory effects on the early passages of cultured cells and inhibition occurring only after long-term treatment, do not present in RPE cells from the PVR membrane. The proliferation of RPE recovered from the inhibition by RA rapidly after the discontinuation of treatment, indicating that a continuous supply of the drug over a long period, i.e., until the breakdown of the blood-retinal barrier has been repaired, is essential for the success of drug treatment of PVR. [ABSTRACT FROM AUTHOR]