The biochemical and immunological characteristics of the chemotactic factor inactivator of group A streptococci (SCFI) were examined. SCFI was extracted from intact M+ bacteria by limited trypsin digestion and purified sequentially by ammonium sulfate fractionation, hydrophobic interaction chromatography, and anion-exchange chromatography. SCFI activity was associated with multiple species as indicated by gel permeation chromatography and DEAE high-pressure liquid chromatography analyses. Polyacrylamide gel electrophoresis of the column-purified protein also demonstrated a high degree of molecular heterogeneity, with most of the material in a 103,000 to 114,000 Mr cluster. SCFI activity was sensitive to destruction by several proteolytic enzymes, and polyclonal antiserum to SCFI was able to neutralize its antichemotactic activity. Two-dimensional immunoelectrophoresis of SCFI indicated that the various copurified species were immunologically cross-reactive and indicated a high degree of antigenic homogeneity within the preparation. Western blot analysis of crude detergent extracts of M+ bacteria identified a major antigenic species corresponding to 135,000 Mr and a less abundant species of 137,000 Mr. SCFI antiserum was not reactive with M protein, and therefore SCFI appeared to be a distinct molecule, despite its close association with the M+ phenotype.