Oral implantations of and salivary IgA activity against a freshly isolated Streptococcus mutans strain and a repeatedly subcultured variant strain were studied in 20 subjects. The variant strain was eliminated at a higher rate than the parent strain. Both strains were eliminated at a lower rate in subjects with high oral levels of indigenous mutans streptococci than in subjects with low levels of these microorganisms. Subjects with high respectively low IgA activity against the parent strain also had high respectively low IgA activity against the variant strain (P less than 0.01). The correlation between IgA activity and clearance rate of implanted bacteria was not statistically significant. However, following both implantations, median values of IgA activity were consistently higher in subjects who had eliminated the implanted strain than in subjects who had not.