Earlier studies have suggested that in adults the n-positive allele of the human IgG2 gene is more productive than the n-negative allele. This superiority was seen to be manifested in IgG2 antibody responses to polysaccharides, in the higher serum concentration of total IgG2 in the n/n than in -/- individuals, and in the higher concentration of n-positive than n-negative IgG2 in heterozygotes. The present study shows that in 1- or 2-year-old children, the concentration of IgG2 was independent of allotype G2m(n), and both alleles of a heterozygote contributed an average of one-half of the total IgG2. On the other hand, the superiority of the n-positive allele was also seen in young children in IgG2 antibody responses induced by the Haemophilus influenzae type b polysaccharide (Hib). The effect of allotype n on antibody responses was evident only when the immunogen was the Hib polysaccharide. When the immunogen was a conjugate of Hib and diphtheria toxoid, the IgG2 antibody responses of n-positive and n-negative vaccinated individuals were almost equal, both in adults and in children.