The mode of topical action of minoxidil (U-10,858, CAS 38304-91-5) ist not entirely clear. The influence of minoxidil on the ultimate differentiation of keratinocytes was investigated. Human keratinocytes (HaCaT-cells) were incubated with minoxidil containing culture medium (10-250 micrograms/ml). Minoxidil inhibited in a concentration dependent manner cell mobility whereas the adhesion area and the cell circumference were increased. The minoxidil treated cultures had a higher desmosome density compared to parallel control cultures and they expressed the suprabasal keratins 1 and 10 (indicating progress in differentiation) earlier and to a larger extent than the controls. Keratins were revealed immunohistochemically and by two-dimensional polyacrylamide gel electrophoresis. The results suggest that minoxidil supports the differentiation of human keratinocytes.