The aim of the investigation was to study the influence of human choriocarcinoma cell culture supernatants (HCS) on the expression of lymphocyte surface molecules. Lymphocytes were stimulated with phytohemagglutinin in culture medium supplemented with 50% HCS. After 12 h and 60 h, fluorescence-activated cell sorter analysis was performed with specific monoclonal antibodies. After 12 h of incubation, the cell surface expression of the classical activation markers CD25, CD69, CD71, CD134 and CD3/HLA-DR, as well as of the apoptosis marker CD95 were significantly suppressed by HCS. Qualitatively similar results were obtained after 60 h. It was concluded that choriocarcinoma-induced suppression might be related to a nearly complete block in the cell cycle at early activation steps. The clinical and biological relevance of this phenomenon for the maternal-fetal tolerance are briefly discussed.