In this report, opsonic activity of hyperimmune rabbit IgG against fibronectin-binding proteins (gal-FnBP A and ZZ-FnBP B) of Staphylococcus aureus is described. Moreover, the action of IgG purified from serum of rabbits immunized with 'combined vaccine' (fibronectin-binding protein A+collagen-binding protein+alpha-toxoid) is shown. The opsonic activity has been studied in an in vitro phagocytosis assay as well as in vivo. Mice which had been infected intraperitoneally with S. aureus strain Cowan 1 pretreated (opsonized in vitro) with specific anti-FnBPs IgG were able to eliminate the staphylococci from the peritoneal cavity and liver more rapidly than controls. Also, clearance from the bloodstream of intravenously injected S. aureus Cowan 1 as well as S. aureus U320, opsonized with IgG anti-FnBPs or anti-FnBP+CnBP+alpha-toxoid, was more effective than observed in control groups. In other in vivo experiments it was shown that mice passively immunized with hyperimmune IgG anti-FnBP (one or two doses, intravenously) before challenge with S. aureus Cowan I eliminated the bacteria better than controls injected only with preimmune IgG.