In a series of 61 consecutive patients undergoing heart, heart and lung, and lung transplantation, 24 patients were known to be cytomegalovirus (CMV) antibody negative on the day of transplantation. Enzyme linked immunosorbent assays (ELISA) for CMV IgG were performed on donor samples on the day of operation. In 16 of the 24 susceptible patients the test was negative and the only preventive measure taken was the use of blood and blood products from CMV-antibody negative blood donors. None of these patients acquired primary infection with CMV. In another six patients the donor serum was found to contain CMV specific IgG, and in these patients, including one heart and lung transplant recipient, prophylaxis with CMV specific hyperimmune globulin was given. All six patients developed CMV IgM antibodies and in five there was an associated but clinically mild illness. None of these patients required treatment. In the remaining two patients ELISA tests on the donor sera gave equivocal results and hyperimmune globulin was withheld. Both patients developed primary CMV infection of greater severity than those given hyperimmune globulin and one required treatment. Reference tests confirmed that the donor sera contained CMV antibodies. Primary CMV infection in susceptible patients after heart transplantation can be avoided by the use of screened blood and blood products where the organ donor is seronegative to CMV and it can be improved by the use of prophylactic hyperimmune globulin where the donor is CMV antibody positive.