Recent reports showed a positive therapeutic effect of foscarnet on Kaposi's sarcoma (KS) in HIV-positive patients. We investigated the therapeutic effect of foscarnet on KS-derived cell lines. Three KS spindle cell cultures of HIV-positive patients as well three fibroblast cell cultures of HIV-positive as HIV-negative patients were established. The influence of different foscarnet concentrations (50, 100, 200, 400, 600, 1000 microg/ml) on these cell lines were analysed. There was no significant change in fibroblast cell numbers suggesting that there is neither a proliferative nor an antiproliferative effect of foscarnet on these cell lines. Furthermore, incubation of the three KS-derived cell lines of HIV-positive patients with different foscarnet concentrations did not change KS-cell number significantly.