Sera from patients affected either by the classic/Mediterranean form (5 patients) and human immune deficiency virus (HIV)-associated (12 patients) form of Kaposi's sarcoma (KS) were compared in supporting endothelial cell in-vitro proliferation. Healthy blood donors (15) and a group of 7 HIV-positive drug addicts with no dermatological involvement were used as control groups. Endothelial cell growth was assessed by means of a [3H]thymidine incorporation and by an optical density direct cell-counting assay. Our results indicate that, at the highest concentrations, sera obtained from patients affected by KS with no sign of HIV infection induced significantly higher levels of endothelial cell proliferation when compared with HIV-associated KS and with normal controls. This growth-promoting activity was apparently cell selective, being present in endothelial cell but not in fibroblast cultures.