Rabbit antisera raised against the product of ORF UL 80 of human cytomegalovirus (CMV) genome (HindIII L fragment of AD169 strain) as well as IgM from acutely infected patients recognize an antigen of Mr 38 kDa. In the viral particle this antigen is bound via S-S bridge to a lower Mr compound to form a final complex of 62 kDa that is also recognized by rabbit antisera as well as patients' IgM. P38 is present both in the nucleus and in the cytoplasm of CMV-infected cells starting from 24 h p.i. and increasingly thereafter. Immunoelectron microscopy revealed that this antigen is mainly associated with the internal portion of viral capsids both in the nucleus and in the cytoplasm.