This study was designed to further characterize the fibrinogen-binding properties of Actinomyces pyogenes. The fibrinogen-binding capacities of a selected A. pyogenes culture could be significantly enhanced by heat pretreatment (60 degrees C, 1 h) of the bacteria. The fibrinogen-binding site seemed to be a protein which was specific for fibrinogen. In phagocytosis studies, binding of fibrinogen to A. pyogenes significantly increased the phagocytic capacity of bovine polymorphonuclear leucocytes.