Up to 30% of patients with acquired immunodeficiency syndrome (AIDS) suffer from Kaposi's sarcoma (AIDS-KS). The histogenesis and neoplastic nature of this tumor is still controversial. We have established cell cultures of KS biopsies from 7 patients with AIDS. All donors were seropositive for the human immunodeficiency virus I (HIV-I), cytomegalovirus (CMV) and hepatitis B virus (HBV). The tumors were histologically shown to be KS. Cell cultures derived from these tumors all expressed the endothelial cell marker BMA 120 antigen. Most of these cultures were positive for acetylated low-density lipoprotein (acLDL) uptake and alkaline phosphatase (AP) expression, and negative for factor-VIII-related antigen (FVIII-RAg). The staining pattern was heterogeneous with respect to number of endothelial cell markers expressed in each culture. We conclude from subcloning experiments that the cultured cells cease to express acLDL receptor and AP, but not the antigen detected by the monoclonal antibody (MAb) BMA 120. The cells grew well in culture up to 50 passages and showed a fibroblast-like morphology. Assays performed to investigate their degree of malignancy revealed a significantly increased passage number under reduced serum conditions as compared to normal fibroblasts but no tumor formation in nude mice. Neither HIV, HBV nor CMV sequences were found in any of the cell lines tested. We conclude that AIDS-KS is an endothelial-cell-derived neoplasm of low malignancy and that HIV, HBV and CMV are not directly involved in its genesis.