To define better the human immune response to individual structural proteins of human cytomegalovirus (HCMV), 55 human sera with different IgG and IgM titres were studied for their reactivity with HCMV structural polypeptides separated by SDS-PAGE and electrotransferred to nitrocellulose paper. The results obtained showed that antibody titres detected by immunoassay correlate with the intensity and the number of polypeptides reacting by immunoblotting (IB). The IB profiles of HCMV polypeptides reacting with different sera having the same antibody titres show considerable variation. Sera with high levels of IgG antibody and that are IgM-positive frequently react with 155, 149, 82.5, 74.5, 67, 57, 55, 38.5, and 28 kD polypeptides; all these sera react with 155, 67, 57, 55, 38.5 kD polypeptides. Sera with high levels of IgG antibody but that are IgM negative frequently react with all these polypeptides, with the exception of 149 and 74.5. Only 155 and 28 kD polypeptides were recognized by all sera of this group. The sera with moderate levels of IgG antibody preferentially recognize 155, 110, 82.5, 62, 55, 38.5 and 28 kD polypeptides. The sera with low levels of antibody reacted especially with 155 and 62 kD polypeptides. IgM antibody seems to recognize preferentially 155, 110, 67, 57, 55, 38.5 kD polypeptides.