When phagocytosis of Staphylococcus aureus by human polymorphonuclear leukocytes (PMN) takes place on the surface of cultured human endothelial cells, the endothelial monolayers are damaged by lysosomal enzymes that are released by the PMN. Because PMN can phagocytose opsonized as well as unopsonized staphylococci on an endothelial surface, we studied the role of bacterial opsonization in the damage caused to the endothelium. Phagocytosis of unopsonized S. aureus was accompanied by greater damage (expressed as the percentage of the endothelial cells detached from the culture plates) of the monolayers than was phagocytosis of opsonized S. aureus: 52 +/- 10% and 24 +/- 7%, respectively, after 30 min of phagocytosis and 73 +/- 5% and 50 +/- 6%, respectively, after 60 min of phagocytosis. When correlated to the amount of phagocytosis, this difference was even greater (uptake was 35 +/- 4% for unopsonized S. aureus and 56 +/- 5% for opsonized S. aureus after 30 min and 42 +/- 3% and 60 +/- 5%, respectively, after 60 min). Total release of lysozyme and myeloperoxidase and generation of superoxide anion were the same during phagocytosis of opsonized or unopsonized staphylococci. Adherence of PMN to the endothelial cells was greater during phagocytosis of unopsonized S. aureus: 42 +/- 4% verus 27 +/- 3% during phagocytosis of opsonized staphylococci. Possibly, increased adherence of the PMN resulted in a locally higher concentration of enzymes which induced more damage. We conclude that opsonization of bacteria not only improves bacterial uptake, but also protects bystander cells from damage by the phagocytosing PMN.