In an attempt to prevent primary cytomegalovirus infection after marrow transplantation, we randomly assigned 97 patients who were seronegative for antibody to cytomegalovirus before transplantation to receive one of the following: (1) both intravenous cytomegalovirus immune globulin and seronegative blood products (23 patients); (2) seronegative blood products alone (28 patients); (3) globulin alone (22 patients); or (4) neither treatment (24 patients). Patients not assigned to receive seronegative blood products received unscreened blood products from random donors. The incidence of cytomegalovirus infection according to study group among patients in the study for at least 62 days was 5 percent, 13 percent, 24 percent, and 40 percent, respectively. Among 57 patients with seronegative marrow donors, those who received seronegative blood products had significantly less infection (1 of 32) than those who received standard blood products (8 of 25, P less than 0.007). In contrast, the use of seronegative blood products did not appear to prevent cytomegalovirus infection among patients with seropositive marrow donors. The possibility that cytomegalovirus immune globulin as used in this study can prevent cytomegalovirus infection or ameliorate cytomegalovirus disease was not confirmed, and it cannot be recommended for routine use without additional study.