with exogenous cytomegalovirus DNA. The high sensitivity of the polymerase chain reaction makes exclusion of specimen contamination very difficult.89 It is thus impossible on the basis of the evidence currently available to say whether the polymerase chain reaction can detect cytomegalovirus DNA in peripheral blood mononuclear cells collected from people with latent infection. If the polymerase chain reaction is adequately sensitive for this purpose it may prove valuable in studying the nature of latent cytomegalovirus infection and screening patients for this infection.