Serum levels of complement components C3, C4 and factor B were measured in twenty-five patients with Staphylococcus aureus bacteraemia. The levels of C3 were depressed in nine patients, five of whom also had low C4 levels, indicating activation of complement via the classical pathway. Two patients with low C3 levels also had low factor B levels, one of these being in association with a low C4 level, which indicates activation of both the classical and alternative pathways, the latter being via the C3b feedback cycle. The protein A content of the S. aureus cultures, as shown by the indirect haemagglutination titre, was high in nine patients with C3 hypocomplementaemia. There was some correlation between the presence of a high protein A content of the S. aureus culture and a low serum C3 level in the patient. Some clinical evidence of immune complex disease was found in three patients. Complement activation in S. aureus bacteraemia is most likely due to complex formation resulting from the interaction between the Fc portion of IgG and staphylococcal protein A. These complexes may also result in clinical symptoms of immune complex disease.