The effects of platinum complexes on human fibroblasts and keratinocytes have been investigated. Cytotoxic effects were observed at approximately 4 microM (cisplatin), 10 microM (carboplatin) and 15 microM (JM9) when cells were grown continuously in these agents. When applied topically to hairless mouse skin in vitro carboplatin was shown to penetrate the skin and give levels where a cytotoxic effect would be apparent. Using an in vivo model of epidermal hyperproliferation in the mouse, an antimitotic effect of a 1% and 5% topical carboplatin preparation was demonstrated. These results suggest that topical platinum complexes are of potential in the therapy of hyperproliferative skin diseases.