Cyclosporin A, which is a specific immunosuppressive compound, has recently been demonstrated to be of significant benefit in the treatment of psoriasis. Because hyperplasia is a major feature of psoriasis, we have investigated whether this drug acts directly to inhibit keratinocyte growth. We have determined the concentration range of cyclosporin in the epidermis of psoriatic patients undergoing cyclosporin therapy and the effect of this concentration range on the growth of cultured keratinocytes. After 7 days of treatment, psoriatic involved epidermis contained 1.1 +/- 0.3 ng cyclosporin/micrograms DNA. Based on tissue wet weight this is approximately 2.8 micrograms cyclosporin/ml. This value was 10 times that of trough blood samples. On day 3 of treatment, involved epidermis contained 7 times more cyclosporin than scale, while on day 7 this ratio was reduced to 2. This suggests that cyclosporin was initially associated with the lower layers of the epidermis and distributed upward with time. Exposure of adult human keratinocytes, cultured on collagen in the presence of human serum, to cyclosporin (0.1-30 micrograms/ml, 0.4-13-fold higher than lesional cyclosporin) for 2-6 d had no effect on the rate of incorporation of thymidine into DNA. Cyclosporin (1-30 micrograms/ml) also had not effect on the reinitiation of DNA synthesis of quiescent cells subsequent to the reintroduction of serum. In contrast, cyclosporin (1-10 micrograms/ml) inhibited growth of keratinocytes cultured on plastic culture dishes in serum free media (MCDB 153). For a given dose of cyclosporin, cell associated drug was 2-3 times greater in serum free compared to serum containing media. This may contribute in part to the sensitivity of keratinocytes in serum free media to growth inhibition by cyclosporin. These data demonstrate that human epidermis contains a high concentration of cyclosporin after oral administration, and that, under certain conditions, concentrations of cyclosporin within the range found in vivo can inhibit growth of cultured keratinocytes. Because cyclosporin regulates lymphocyte function in vivo and in vitro, the demonstrated antiproliferative effects of cyclosporin on psoriatic keratinocytes in vivo may be due to inhibition of a mononuclear leukocyte-derived keratinocyte growth factor(s) in combination with direct inhibition of keratinocyte growth.