Plasma anti-pneumococcal polysaccharide antibody activity (serotypes 3, 6a and 23) was determined in samples from 15 otitis prone children, at 30 months of age, and compared with age matched control children and adults. A recently developed enzyme immunoassay, using IgG subclass specific monoclonal antibodies for analysis of pneumococcal antibody activity of different IgG subclasses was employed. Adult sera always contained the highest antibody concentrations, almost exclusively of the IgG2 subclass. Children, on the other hand, had higher amounts of IgG1, significantly exceeding those of the adults, healthy children having higher IgG1, as well as IgG2 values than otitis prone children. The most significant differences were seen with type 6a, less so with type 23 antibodies. No differences in IgG1 anti-type 3 pneumococcal activity were observed between the children and the adults. These findings support the concept that pneumococcal antibody activity is confined to IgG2 and, in addition, it was found that sera from children contain antibodies of both IgG1 and IgG2 subclasses.