Staphylococcus aureus antigen was identified within the glomeruli of a patient with acute bacterial endocarditis and diffuse glomerulonephritis. Routine immunofluorescence had revealed only granular deposits of complement (C3). C3 activator and C4 were not present. Direct immunofluorescence studies with a specific anti-Staphylococcus aureus conjugate were positive. Electron microscopy showed subepithelial and intramembranous electron-dense deposits. Eluates of the kidney did not contain anti-S. aureus antibody. The absence of immunoglobin suggests that the toxic action of S. aureus antigens may activate complement and cause glomerular injury and that immune complexes are not essential for the production of glomerulonephritis in this entity.