It is questionable as to whether a low serum concentration of one of the IgG subclasses identifies a disease state. A low IgG(1) concentration is found in primary or secondary immunodeficiency states but does not occur in isolation. Low IgG(2) concentration is associated with an increased risk of bacterial infections but only in some individuals and not in others. Isolated IgG(3) and IgG(4) deficiency have not been convincingly demonstrated. Therefore, the isolated finding of low concentrations of one or more IgG subclass does not identify individuals at risk. In contrast, the finding of low serum concentrations of antibodies to specific bacterial antigens (Haemophilus influenzae type B, pneumococcus, tetanus and diphtheria) does identify individuals at risk and these measurements should be used in preference to IgG subclass measurement.