Immunoglobulin G (IgG) subclass levels were determined in 393 serum samples of 329 healthy children from birth to 16 years of age, 24 healthy adults and 20 pairs of mother-cord blood by radial immunodiffusion. Age-normal percentile values for IgG1, IgG2 and IgG3 were calculated for age-groups up to 16 years and for adults. The broad range of IgG4 values in children did not permit calculation of reference values.