The effect of several naturally-occurring retinoids and 13-cis-retinoic acid on the proliferation of cultured bovine retinal pigment epithelial (RPE) cells was investigated. None of the retinoids tested were toxic to the cultures and all, except retinylpalmitate, inhibited cell proliferation when given for more than 3 days. The relative potencies of the retinoids were; all-trans-retinoic acid>13-cis-retinoic acid>all-trans-retinol≅all-trans-retinaldehyde. Uptake of retinoic acid by cultured RPE cells was 10-fold less than the uptake of retinol. Although retinoic acid-treated cultures showed strong density-dependent growth inhibition, cellular proliferation was inhibited more in sparse cultures than in dense ones. Retinoic acid did not significantly inhibit the proliferation of first passage bovine or rabbit RPE cells, but partially inhibited the proliferation of first passage human RPE cells. The sensitivity of all these cultures to growth inhibition by retinoic acid increased in subsequent subcultures, yet there was no effect of passage number on retinoic acid uptake. This study demonstrates that RPE cell proliferation can be inhibited by retinoic acid but the sensitivity of these cells to the retinoid's effects are modulated by incubation time, in vitro aging, and cell density.