The interactions of C3a anaphylatoxin with vascular endothelium were studied in vitro using human endothelial cells in culture and 125I-labelled human C3a. Cultured endothelial cells took up 125I-C3a in a time- and concentration-dependent manner and inactivated it. Uptake was not associated with binding to specific receptors since the amount of radioactivity accumulated by the cells was not influenced by treatment with excess unlabelled peptide, metabolic inhibitors or by low temperature. Further, we observed that uptake was not saturated during 90 min of incubation or within the concentration range of C3a tested (10(-9)--10(-6) M). C3a was taken up more rapidly than other labelled, less basic compounds, including Tyr5-bradykinin, lysozyme and albumin. Examination of the cells by autoradiographic electron microscopy revealed labelled material within the cell cysoplasm but not within specific intracellular structures, such as vesicles or vacuoles. C3a was partially inactivated after incubation with endothelial cells for 15 min, but some spasmogenic activity was retained even after 90 min incubation. Since the peptide is readily inactivated by the cells, the radioactivity in the cytoplasm may be inactive C3a and possibly C3a fragments. The combination of uptake and inactivation of C3a by endothelial cells may be an effective means of removing the peptide from circulation.