Evaluate humoral immunity; monitor therapy in IgG myeloma; quantitate IgG; evaluate patients, especially children and those with lymphoma, with propensity to infections. In congenital hypogammaglobulinemia, the IgG is less than 200 mg/dL by 6 months of age. Acquired hypogammaglobulinemia may occur at any age and has IgG concentrations less than 500 mg/dL. IgG concentrations may also be decreased in combined cell-mediated and antibody immunodeficiencies.