S-protein, also named vitronectin, is a multifunctional glycoprotein with molecular weight (MW) of about 75 kDa and a serum concentration of 0.14-0.60 mg/ml. It is synthesized mainly in the liver, but synthesis has also been found in monocytes/macrophages. We used human umbilical vein endothelial cells (EC) which were incubated with agarose beads, an activator of the alternative complement pathway. By radioimmunoassay (RIA) based on monoclonal and polyclonal S-protein antibodies, we detected S-protein on harvested agarose beads. The time-dependent increase in the amount of S-protein was significantly reduced by the presence of cycloheximide (10 micrograms/ml) in the cell cultures. We also found a strong binding of S-protein antibodies to agarose beads preincubated in native serum, which was strongly reduced (70-80%) by inactivation of the alternative complement pathway (50 degrees C, 20 min). Our results show that EC synthesize S-protein in vitro.