Normal human and rabbit sera when incubated with
Staphylococcus aureus
inhibit the adsorption of bacteriophages. The bacteriophage adsorption was also inhibited by separated normal immunoglobulin M (IgM), F(ab′)
2
, and Fab-fragments of IgG. No inhibition was obtained with myeloma IgG or Fc-fragments of normal human and rabbit IgG. The results indicate that the serum inhibition of bacteriophage adsorption to
S. aureus
is not due to a binding of IgG to protein A on the surface of
S. aureus.