Specific binding of the 125I-labeled human S protein (vitronectin) which has been shown to be identical with serum-spreading factor, was observed with group A, C, and G streptococci as well as with Staphylococcus aureus and Escherichia coli. The specific binding of S protein to group A, C, and G streptococci was high, whereas the binding to S. aureus and E. coli cultures was moderate. In contra...