Post-transfusion CMV infection most frequently results in asymptomatic seroconversion. Among immunocompetent patients only seronegative pregnant women require such products because of the risk of fetal CMV infection. In selected groups of immunocompromised patients, significant disease can occur. It is desirable to provide blood and blood components with reduced CMV risk to the following patients: seronegative infants weighting less than 1200 g at birth, seronegative bone marrow transplant patients who receive marrow from seronegative donors and seronegative renal transplant patients receiving kidneys from seronegative donors. Heart and liver transplantation seronegative patients may receive seronegative blood if the donor is seronegative. CMV--seronegative HIV infected cases may also be transfused with CMV--seronegative blood.