Antibodies to streptococcal extracellular products were detected in rabbit sera by passive hemagglutination tests as early as 1 week after intravenous injection of live group A streptococci (strain C203S). Using these antibodies in immunoadsorbent columns, the authors prepared an antigenic fraction from crude concentrates of cellular products. The specific activity of this fraction in sensitizing glutaraldehyde treated erythrocytes for passive hemagglutination tests and in absorbing passive hemagglutination antibodies from immune sera was increased by 80 fold and at least 24 fold, respectively, as compared with crude extracellular products. The fraction has been found to contain at least three serologically active antigens, which are of considerable interest because: they gave rise to early and consistent immune responses both in humans and in experimental animals; they appear so far to be produced only by beta hemolytic streptococci; and antibodies to these antigens were present in high titers in patients with acute rheumatic fever. The data suggest that passive hemagglutination antigens may be distinct from those extracellular enzymes and hemolysins ordinarily employed in streptococcal serological studies.