The functional activity of naturally acquired IgG and IgM with specificity for the capsular polysaccharide of type III, group B Streptococcus (III-GBS) was compared. Sera collected during convalescence from 14 infants who had developed specific antibodies following III-GBS infection were employed for separation of IgG from IgM by ion-exchange chromatography. Bactericidal killing of III-GBS was similar for IgG- and IgM-rich samples (37 and 42%, respectively) in reactions containing a mean of 1.0 or 1.9 μg/ml of III-GBS specific IgG or IgM antibody. Purified IgA lacked opsonophagocytic activity for III-GBS. These results indicate that III-GBS-specific IgG and IgM antibodies in infant sera promote opsonophagocytosis at low concentrations, and that their functional capacity is comparable.