Two distinct peaks were obtained when human parotid saliva was separated on a Sepharose 2B column. The bacterial agglutinating activity was concentrated to the void volume fractions whereas the IgA was found in the beginning of the second, large peak. Unfractionated saliva as well as the pooled agglutinin fractions, or a mixture of agglutinin and IgA, all induced the aggregation of KPSK2, a Streptococcus mutans serotype c strain. By adding anti-human-IgA antiserum to the whole saliva or to the mixture of agglutinin and IgA, the aggregation reaction could be eliminated. In order to achieve this effect the agglutinin and IgA had to be mixed prior to the addition of anti-IgA. Addition of anti-IgA antiserum to the agglutinin fraction only did not impair the aggregation of bacteria. The homologous reactions with anti-IgG antiserum did not give any inhibition effect. However, when human IgG was added to the saliva, or to the agglutinin, before the addition of anti-IgG, the aggregation of KPSK2 was again impaired. The data in this paper indicate that the agglutinins and the IgA antibodies in saliva may be normally associated with each other.