There is controversy about whether Cyclosporin A (CsA) affects antigen-presenting cell function. Within the skin, Langerhans cells (LC) are very potent antigenpresenting cells. We investigated the effect of CsA on alloantigen presentation by human LC using the in vitro mixed skin-cell lymphocyte reaction (MSLR). MSLR (6 day cultures) were performed in round-bottomed microplates and lymphocyte proliferation was assessed by 3H-thymidine incorporation during the final 18 h of culture. When CsA was added into the wells a dose-dependent inhibition of T-cell proliferation occurred. Similar results were obtained when crude or LC-enriched epidermal cells (EC) were incubated for 2 h in the presence of CsA and extensively washed. The inhibition caused by CsA treatment of EC was not overcome by the addition of indomethacin. However, when CsA-treated EC were added to a fresh MSLR, T-cell proliferation was impaired. Furthermore, supernatants from CsA-treated EC, that had been kept for 6 days in culture medium, were able to inhibit the T-cell proliferative assay. These supernatants were found to contain CsA by a radioimmunoassay. From these results, it is clear that inhibition of MSLR obtained after CsA pulsing of EC suspensions can be explained by a release of the drug into the supernatant and thus by a direct effect on T cells. These findings contrast with recent reports showing a direct effect of CsA on human LC function.