Binding of 125I-labelled alpha 2-macroglobulin (alpha 2M) to Actinomyces pyogenes was investigated. The binding of alpha 2M proved to be time dependent, saturable, and could be inhibited by unlabelled alpha 2M, but not by fibrinogen, haptoglobin, fibronectin, or albumin. The alpha 2M binding site was sensitive to treatment with proteolytic enzymes and heat, indicating its protein nature. The dissociation constant (Kd) was 4.447 x 10(-9) M, and the number of binding sites per bacterial cell was calculated to be 5200. The kinetic analysis indicated an homogeneous population of binding sites. Binding of alpha 2M to the surface of A. pyogenes had no significant influence on the phagocytosis of the bacteria by polymorphonuclear leucocytes.