IgG subclasses have been recognized since the early 1960s. Four such subclasses, designated IgG1, IgG2, IgG3, and IgG4, are known to exist. Approximately 65 to 70% of the total circulating IgG in normal persons is of the IgG1 subclass. IgG2 constitutes 20 to 25% of circulating IgG, and IgG3 and IgG4 each represent less than 10%. Deficiencies in the various IgG subclasses have been detected in adults and children with common variable hypogammaglobulinemia as well as in those with relatively normal total IgG levels. An important issue facing clinicians today is to determine what, if any, therapeutic implications are associated with demonstration of an IgG subclass deficiency.