Human cytomegalovirus (CMV) has at least 15 structural polypeptides which are immunogenic during natural infection. In this report we used immunoblotting to study the reactivity of each polypeptide with the four IgG subclasses present in CMV antibody-positive human serum obtained from both convalescent patients and from patients with serological evidence of active CMV infection. The results showed that in the serum of patients reactivity to several CMV polypeptides is mostly due to all four IgG subclasses, with IgG3 being the most prevalent followed by IgG1. Intermediate molecular weight polypeptides (namely p66, p61, p55, p49, p38) appear to be the major antigens responsible for the preferential induction of the IgG3 response. During convalescence the reactivity of all four IgG subclasses is evident only against a structural polypeptide of 150 kD molecular weight (MW), while intermediate MW polypeptides show variable IgG subclass reactivity. The presence of IgG3 to one or more polypeptides of intermediate MW might be of diagnostic interest.