The influence of human plasma, serum, purified fibrinogen, and fibronectin on Staphylococcus aureus intraperitoneal infection in non-immune mice was studied. Mouse mortality was used as a measure of staphylococcal virulence. Both human plasma and serum were shown to enhance the virulence of S.aureus strain E 2371 and strain E 2476 when added to the bacteria before challenge. This effect of serum was unaffected by storage for 24 h at 37 degrees C or complement-inactivation for 1 h at 56 degrees C. Purified fibrinogen and fibronectin did not influence the S. aureus virulence. It is suggested that the effects of plasma and serum described here might play a role in the establishment of S.aureus infections in humans.