Streptococci of serological groups A, B, C, G and L were examined for interactions with human colostral IgA. Of 28 A-streptococcal cultures, 12 bound IgA with a mean of 38.7%. The other streptococci had little or no IgA-binding activities. Of the 12 IgA-binding A-streptococcal cultures, 8 contained the M-protein M 4 and 2 the M-protein M 60. The specific binding sites for IgA were heat-sensitive (60 min, 80 degrees C) and susceptible to trypsin and pronase.