A simple microneutralization test for cytomegalovirus (CMV) is presented. Using a laboratory adapted stock virus, definite results were obtained after 10 days. All patients with primary CMV infection showed an antibody rise and/or seroconversion; however, neutralizing antibody appeared only seven weeks after the onset of clinical symptoms. In control patients without evidence of recent primary infection, there was a complete concordance of the presence of complement-fixing and neutralizing antibodies.