The ability of Staphylococcus aureus to adhere to endothelial cells (EC) is a critical step in the development of metastatic infection. The role of complement in S. aureus binding to EC remains uninvestigated. Log-phase S. aureus, expressing minimal capsule, was incubated with serum under various conditions, washed, and then incubated at 37 degrees C for 30 min with cultured human umbilical vein EC (ATCC CRL-1730). Adherence was scored visually after staining with acridine orange. Incubation in 10% heat-inactivated human serum increased adherence to endothelial cells by 488% compared to organisms incubated in buffer. Incubating S. aureus in complement-active normal human serum (NHS) decreased binding to EC by 58% compared to organisms incubated in heat-inactivated serum. The importance of active complement was confirmed by experiments using serum with added EDTA or cobra venom factor, a protein that depletes C3. The expression of capsule by S. aureus strongly interfered with adherence. It has been shown that an important protein for S. aureus adhesion to EC is fibronectin. S. aureus adherence to purified fibronectin increased by 511% after incubation in heat-inactivated serum, compared to that of organisms incubated in buffer. This decreased by 56% in complement-active serum, suggesting that inhibition of S. aureus adherence to EC is due, in part, to complement-mediated diminished binding to fibronectin. Interestingly, when EC were exposed to S. aureus-activated serum and then washed, binding by S. aureus was 234% higher than that of EC exposed to NHS. Thus, complement-activated EC have increased S. aureus binding, while complement on the bacterial surface markedly reduces adherence.