A new method of extracting M protein from streptococcal cell walls has been presented. The extracting agent was guanidine-hydrochloride, a protein denaturant. The crude guanidine extract was further purified by ammonium sulfate and pH 5 fractionation and by hydroxyapatite column chromatography. Three major protein peaks were eluted from the hydroxyapatite column with 0.01, 0.1 and 0.3 M phosphate buffer, respectively. Protein fractions eluted at 0.1 and 0.3 M phosphate concentractions contained antigens that precipitated with homologous M-protein specific antisera, whereas the 0.01 M phosphate fraction had no immunological activity. The fraction eluted with 0.3 M phosphate was electrophoretically homogeneous in sodium dodecyl sulfate-acrylamide gels and elicited the production of bactericidal antibodies in rabbits. The 0.1 M phosphate buffer eluant was electrophoretically heterogeneous and did not elicit the production of bactericidal antibodies in rabbits.