Streptococcus pyogenes produced M protein and fibrinogen-precipitating factor. Purified M protein, regardless of the serologic type, interacts not only with fibrinogen, but also with fibrin lysate and soluble fibrin-monomer complexes. This interaction is similar to the paracoagulation reaction induced by protamine sulfate. However, the mechanism of precipitation of fibrinogen is different for streptococcal M protein since it is an acidic protein. It is hypothesized that the paracoagulation reaction involves neutralization of positive charges on fibrinogen by the anionic sites of M protein.