Background : Calcipotriol(MC903), a new vitamin D, analogue, has been reported to be effective in the treatment of patients with psoriasis. Objective : The purpose of this study is to examine the effects of calcipotriol on proliferation and differentiation of the keratinocytes in monolayer cultures and three-dimensional cultures. Methods. Using moriolayer cultures, we examined morphological changes of keratinocytes and performed (3H)thymidine incorporation after calcipotriol was added into the medium. Using three dimensional cultures, we performed two experiments. one with cultures treated with calcipotriol immediately after the keratinocytes had been exposed to the air and another set of cultures treated with calcipotriol after three dimensional morphogenesis of the kerat.inocytes. We examined morphological changes of keraitinocytes and performed a immunohistochemical study for proliferation differentiation markers. Results : In monolayer cultures, at calcipotriol concentrations of 10 'M 10 'M, keratinocytes be- came larger, more irregular, and flattened in a dose-dependent manner. At 10-9 M-10-6 M, [3Hl thymidine incorporatiorn was decreased dose-dependently as compared to the control culture. In the first experiment using three-dimensional cultures, at 10 M-10 'M, total epidermal layers were thinned. This was assnciated with thinnings of nucleat.ed and horny layers in a dose-dependent manner. In the seconci experiment using three-dimensional cultures, at 10 M-10 M, nucleated layers were thinned in a dose dependent manner, but the horny layer was slightly thickened, as compared to the control culture. Immunohistochemical studies showed a reduction of differentiation markers such as keratin 1, involucrin, filaggrin, loricrin consistent with a thinning of nucleated layers in the epidermal architecture in both experiments. In the basal layer, at 10 M-10 'M, PCNA-positive cells were and BrdU-positive cells were decreased dose-dependently as compared to the control culture. Conclusions : In this study, we demonstrated that at 10-9M - 10-6M calcipotriol inhibited keratinocytes proliferation and stimulated keratinocytes differentiation in a dose-dependent manner. (Kor J Dermatol 1996;34(6): 942-952)