Early antigens (EA) of human cytomegalovirus extracted from cytosine arabinoside-blocked cells infected with 0.01-20 infectious units (IU)/cell were assayed with human serum by electroimmunodiffusion (EID). The number of detectable EA types increased from one to eight as the IU/cell was raised from 0.01 to 10. There was no increase in the number of EA with further increases in IU/cell, with prolonged culture, or when detergent was included in the extraction buffer. At least five of the eight EA gave reactions of identity with late-time antigens (LTA) extracted from unblocked cells at late times postinfection. In studies on a panel of sera from donors who were excreting virus and donors who were not, EID was as sensitive as conventional techniques (complement fixation and indirect hemagglutination for LTA, indirect immunofluorescence for EA) in detection of both types of antibodies from excretors but less sensitive in not detecting low levels of the antibodies in some of the sera from nonexcretors. No consistent relationships were observed between donor virological status and the numbers or types of antibodies to EA and LTA.