An early cytomegalovirus (CMV) antigen was detected with a monoclonal antibody by two-color fluorescent flow cytometry. With the aid of a human diploid fibroblast cell strain, FLOW 2000, infected with the AD169 strain of CMV, the viral antigen and the DNA content of infected or uninfected cells were measured. There was no evidence of change in the cell-cycle distribution of the infected cells. The viral antigen was detected within 30 minutes following virus adsorption at 0.1 and 1.0 plaque-forming units/cells; and the percentage of positive cells increased with time and viral dosage. All stages of the cell cycle were susceptible to viral infection and the average fluorescence was greater than the background fluorescence. Flow cytometry detected the viral antigen earlier than conventional immunofluorescent microscopy and cell culture for CMV cytopathological effect (CPE). Ten bronchoalveolar lavages assayed by flow cytometry and conventional diagnostic procedures demonstrated that flow cytometry might be useful in early diagnosis for CMV infection.