The reactions of human sera (with or without high titers of antistreptolysin O0 and of rabbit or horse immune sera against filtrates of [beta] hemolytic streptococci and derivatives (such as Dick toxin, M antigen, streptolysin O) were analyzed on the basis of a human reference serum producing 8-9 lines of precipitation. Heat stability of the antigens involved and relations to the toxic fractions were investigated. Human (patients'') sera tend to show more lines of precipitation than do animal immune sera. Many of the human antibodies involved were found in the [beta] globulin fraction on electrophoretic analyses.