The serum levels of the four subclasses of IgG to three different antigens--ovalbumin (OVA), pneumococcal polysaccharide (PNC), and avidin--were determined by enzyme-linked immunosorbent assay in three age groups of healthy children: 1.2-2.5 years (group A, n = 20), 6-7 years (group B, n = 21) and 15-17 years (group C, n = 20). The anti-OVA response in all groups was dominated by IgG1, IgG2, and IgG4 antibodies which generally increased in levels from group A to group B, but then declined significantly in group C. In all age groups, the IgG2 and IgG4 as well as the IgG1 and IgG3 responses were correlated; an association was also observed between IgG1 and IgG2 in groups B and C, but not in group A. The anti-PNC response was restricted to the IgG2 and IgG4 subclasses; correlation between these subclasses was seen only in group B children. The more predominant IgG2 antibodies increased dramatically in concentration from group A to group B, and then less remarkably in group C. Occurrence of the IgG1 anti-PNC antibodies was different which resembled the anti-OVA response. Responses to avidin, a protein derived from the same source as OVA, were different from the anti-OVA responses. These were low and confined to the IgG1 and IgG2 subclasses only and mainly to the younger children. A small group (n = 8) of 6- to 7-year-old atopic children who had a history of asthma or eczema appeared to have responses similar to their healthy counterparts.