We investigated the IgG subclass reactivity pattern to early antigens (EA) of human cytomegalovirus (HCMV) in 217 EA-antibody-positive sera from immunocompetent, healthy persons, renal transplant recipients and AIDS patients by immunoblotting. All IgG subclasses are involved in the IgG immune response to HCMV EA. IgG 1 was the major subclass reacting with HCMV EA (with a molecular mass ranging between 23- and 79-kDa) and was present in all sera irrespective of origin. Antibody responses of IgG isotypes 2, 3 and 4 were observed with lower frequency and reactivity, whereas IgG 3 was detectable more frequently and reacted slightly stronger than IgG 2 and 4. The IgG 1 reactivity pattern was similar to that seen with total IgG. In contrast to total IgG and IgG 1, the reactivity of the sub-classes 2, 3 and 4 was not equally distributed among the early polypeptides, but was mainly directed to some of them (79-, 70-, 66-, 43- and 38-kDa). On average primary infections seem to induce a stronger IgG 3 response to the 79-, 70- and 38-kDa proteins than reactivated infections and an increased IgG 1 to the 79-, 70-, 59-, 56- and 50-kDa proteins appeared to be associated with severe disease. Noteworthy was the significantly lower prevalence of IgG 1 antibodies to the 70-kDa protein in human immunodeficiency virus (HIV)-infected individuals when compared to renal transplant recipients and immunocompetent, healthy individuals. Since the IgG 1 immune reaction to this protein occurred five to seven times more frequently in healthy persons and renal transplant recipients, the decreased formation of IgG 1 antibodies to the 70-kDa protein might be a characteristic feature of HIV infection.