The transmission of cytomegalovirus (CMV) by blood transfusion may have a major effect on certain immunocompromised patients. To protect susceptible blood recipients from infection, it is advisable to use blood components from CMV-seronegative donors. However, serologic tests are not capable of indicating which blood component actually harbors infectious virus and can transfer it to the recipient. Therefore, a sensitive method is needed for the detection of the virus itself. There have been three reports on the detection of CMV in healthy volunteer blood donors by the polymerase chain reaction (PCR). CMV DNA was found in all seropositive and most seronegative blood donors. However, many other authors have failed to confirm these data. A highly sensitive and specific PCR assay was developed for the detection of CMV DNA in peripheral blood leukocytes. With this protocol, blood samples from 116 volunteer blood donors were investigated. None of these samples proved to be positive for CMV DNA. In contrast, CMV DNA was detected in 10 of 10 renal transplant patients early in the course of active CMV infection. It can be concluded that the CMV genome copy number in the peripheral blood leukocytes of healthy individuals is beyond the detection limit of current PCR technology.