The concentrations of IgG1, IgG2, IgG3 and IgG4 were determined by electroimmunoassay in 10 pairs of maternal and cord sera and in sera of 162 healthy children, aged 6 weeks to 15 years. Specific rabbit antisera against the IgG subclasses were used. The content of the normal serum pool WHO 67/97 was used as reference. The mean value, standard deviation and normal range of each IgG subclass were calculated for each age group and compared with the adult values. All IgG subclasses were present in cord serum except for IgG4 in those cases where also the maternal serum lacked demonstrable IgG4. The IgG subclasses followed the pattern of total IgG with a fall during the first 3--6 months and a subsequent gradual rise with age. The IgG1 and IgG3 levels rose faster with age than IgG2 and IgG4. Adult levels were not reached before puberty. No IgG4 was detectable in 12--21% of the children above 7 years of age.