IgG, IgM and IgA immunoglobulin classes of antibodies to human cytomegalovirus nuclear antigens (CMNA) were studied by the acid-fixed nuclear binding technique (AFNB) and combined anti-complement immunofluorescence (combined ACIF). In acute cases of infectious mononucleosis (IM) of human cytomegalovirus (HCMV) origin and in the so-called double virus infections (HCMV + Epstein-Barr virus), anti-CMNA IgM antibodies were detected. They were absent from both anti-HCMV positive sera of healthy donors and sera of patients suffering of IM caused by EBV used as controls. The presence of anti-CMNA IgM may thus serve as an additional evidence of acute HCMV infection. Non-complement-fixing IgA classes of the anti-CMNA antibodies were not found in some of the sera gathered during the acute phase of IM of EBV origin: in one fourth of the HCMV seropositive donors and in a number of late serum samples. But non-complement-fixing and complement-fixing anti-CMNA components of the IgG class were detected.