Human cytomegalovirus (HCMV) infection in immunocompromized hosts sometimes occurs as a result of reactivation. Cells of the monocyte-macrophage linkage are suggested to be a site of latency and persistence for HCMV. The human monocytic cell line U937 was infected with the AD169 strain and a clinical isolate of HCMV. The expression of surface antigens on the cells was assessed by flow cytometry. The polymerase chain reaction (PCR) was used to detect viral DNA from infected cells. CMV immediate early antigen, early antigen, and late antigen (LA) were detected from both clinical isolate- and AD169-inoculated U937 cells by flow cytometry. CMV DNA which code major immediate early gene (US3) and LA gene (US14) were detected from the clinical isolate-inoculated U937 over a period of 31 days as tested by PCR. These U937 cells proliferated as well as uninfected U937 cell, but only a small number of AD169-inoculated U937 cells survived after 14 days of inoculation. Interleukin-2 activities were detected in the media on days 24-40 after inoculation with AD169. This chronic CMV infection model of U937 might be utilized to study the mechanisms of persistence and reactivation.