The aim of this study was to determine the effects of 13-cis-RA, all-trans-RA, and acitretin on the proliferation, lipid synthesis, and keratin expression of human sebocytes in vitro and to elucidate possible mechanisms of retinoid action on sebaceous glands at the cellular level. It was found that 13-cis-RA and all-trans-RA decreased sebocyte proliferation in a dose- and time-dependent manner, with a 13-cis-RA – IC₅₀ of 10^-5 M (after 7 d) and 10^-6 M (after 14 d) and an all-trans-RA- IC₅₀ of 10^-7 M (after 14 d; no IC₅₀ after 7 d). Acitretin inhibited sebocyte proliferation only at 10-s M. Furthermore, 13-cis-RA was the most potent inhibitor of acetate incorporation into lipids, which indicated lipid synthesis (48.2% reduction), followed by all-trans-RA (-38.6%), and by acitretin (-27.5%). All retinoids tested markedly decreased the synthesis of triglycerides, wax/stearyl esters, and free fatty acids in cultured sebocytes, whereas squalene synthesis remained uninfluenced and cholesterol synthesis slightly increased. On the other hand, keratin 5 -was down-regulated, keratin 17 was up-regulated, and the expression of keratin 13 was virtually unaffected by all retinoids tested. Keratins 6 and 16 were down-regulated by 13-cis-RA and by all-trans-RA, keratin 14 was down-regulated by 13-cis-RA only, and keratin 19 was up-regulated by all-trans-RA. These investigations indicate that 13-cis-RA and, to a lesser extent, all-trans-RA are potent inhibitors of both cell proliferation and lipid synthesis in human sebocytes in vitro, whereas acitretin only decreases lipogenesis in this model. In addition, retinoids may modify tile-differentiation of sebocytes in Vitro by modulating Keratin expression. Models of cultured human sebocytes are useful tools for further investigations on the sebaceous gland and its activity at the cellular level.