An improved counterimmunoelectrophoretic (CIE) procedure for detection of antibodies to human cytomegalovirus (CMV) is described. The procedure, which uses high ionic strength buffer and concentrated antigen, is as sensitive and specific as complement fixation (CF), indirect hemagglutination (IHA), and anti-complement immunofluorescence (ACIF) but requires concentration of some sera. In studies on sera from 118 normal blood donors, the percent correlation of CIE results with those by CF, IHA, and ACIF were 97.5, 98.3, and 98.3, respectively, and exceeded slightly the correlations among the latter three procedures. The appearance of precipitation immediately or early after a CIE run and of more than one line of precipitate was indicative of high titers by the other procedures, while single lines of late precipitation occurred with most low titer sera but also with many of those of high titer.