One thousand consecutive units of blood, on which donor data could be obtained, were titered for the presence of complement‐fixing (CF) antibodies to the AD‐169 strain of cytomegalovirus. Twenty‐one per cent of the units had a positive titer at a significant level. Blood obtained through commercial sources had a distinctly higher incidence of specific antibody than that obtained from a predominantly student population. Because of the high incidence of CF antibodies to the cytomegaloviruses, a decision to exclude blood from use in transfusion would seriously affect the already critical blood shortage. However, the fact that use of such blood for transfusion is associated with significant danger (especially in certain critical patient populations) makes it important that further studies be carried out to provide the data necessary for a wise dec