Serum IgG subclass levels were measured by an indirect competitive immunoenzymatic assay with monoclonal antibodies in 61 adult patients of different geographic origins affected either with the lymphoadenopathy syndrome (LAS, 46 cases) or with the acquired immune deficiency syndrome (AIDS, 15 cases). In spite of considerable variations from patient to patient, IgG-1 and IgG-3 levels were higher than in normal Caucasians, with IgG-1 levels much more elevated in LAS than in AIDS patients. In Caucasians with AIDS or LAS, IgG-4 levels were low. IgG-2 levels tended to be low and correlated negatively with IgG-1 levels. IgG subclass imbalances were especially striking in patients with lymphoid interstitial pneumonitis. The results suggest that the IgG increase predominantly affects the less T-dependent subclasses. The low levels of the more T-dependent isotypes do not appear to play a clear role in the occurrence of pyogenic infections in certain patients.