The response of cultured human prostatic epithelial cells to vitamin A was measured by clonal growth assay in serum-free medium. Retinoic acid at 3 nM or higher inhibited the proliferation of cell strains derived from normal, benign hyperplastic and malignant tissues, while lower levels (0.03 nM) were stimulatory. Reduced proliferation induced by retinoic acid was accompanied by a marked change in morphology, as intercellular adhesion decreased. In conjunction, the expression of keratins 8 and 18, associated with the differentiated luminal phenotype of prostatic epithelia, was increased. In post-confluent cultures, retinoic acid prevented the appearance of keratin 1, which accompanied the development of a squamous phenotype by cells maintained under these conditions. The findings of this study indicate a role for vitamin A as a modulator of the growth and differentiation of prostatic epithelial cells.