We have previously shown that fibrin can act to contain microorganisms and prevent early septic death in experimental peritonitis. However, this trapping eventuates in abscess formation. Fibrinogen, the precursor molecule of fibrin, is known to possess binding structures for some pathogenic organisms. We compared the extent of incorporation of various aerobic and anaerobic bacteria as well as polystyrene latex microspheres into fibrin clots. Similar numbers of organisms and microspheres were incorporated into either noncontracted or contracted fibrin clots. Detailed comparisons of the binding of Escherichia coli or Staphylococcus aureus to human fibrinogen were then made. The addition of 111:B4 lipopolysaccharide did not inhibit incorporation of E. coli 0111:B4 into either type of fibrin clot. With initial inoculum sizes of 10(6) to 10(8) colony-forming units (CFU)/ml, S. aureus was better incorporated into contracted fibrin clots (P less than 0.01) than was E. coli, possible evidence for an easily saturable receptor mechanism. We concluded that microorganisms are incorporated into the polymerizing fibrin matrix in the same fashion as are inert particles of similar size, irrespective of external chemical structure. Adherence of bacteria to fibrinogen or polymerizing fibrin did not appear to represent a specific bacterial virulence factor, more likely representing an effective host defense mechanism of broad specificity.