Using a panel of 143 strains classified according to a novel taxonomic system for oral viridans-type streptococci, we reexamined the ability of oral streptococci to attack human immunoglobulin A1 (IgA1) molecules with IgA1 protease or glycosidases. IgA1 protease production was an exclusive property of all strains belonging to Streptococcus sanguis and Streptococcus oralis (previously S. mitior) and of some strains of Streptococcus mitis biovar 1. These are all dominant initiators of dental plaque formation. Degradation of the carbohydrate moiety of IgA1 molecules accompanied IgA1 protease activity in S. oralis and protease-producing strains of S. mitis biovar 1. Neuraminidase and beta-galactosidase were identified as extracellular enzymes in organisms of these taxa. By examination with enzyme-neutralizing antisera, four distinct IgA1 proteases were detected in S. sanguis biovars 1 to 3, S. sanguis biovar 4, S. oralis, and strains of S. mitis, respectively. The cleavage of IgA1 molecules by streptococcal IgA proteases was found to be influenced by their state of glycosylation. Treatment of IgA1 with bacterial (including streptococcal) neuraminidase increased susceptibility to protease, suggesting a cooperative activity of streptococcal IgA1 protease and neuraminidase. In contrast, a decrease in susceptibility was observed after extensive deglycosylation of the hinge region with endo-alpha-N acetylgalactosaminidase. The effector functions of IgA antibodies depend on the carbohydrate-containing Fc portion. Hence, the observation that oral streptococci may cleave not only the alpha 1 chains but also the carbohydrate moiety of IgA1 molecules suggests that the ability to evade secretory immune mechanisms may contribute to the successful establishment of these bacteria in the oral cavity.