The Epstein-Barr virus (EBV) has occasionally been noted to cause transfusion-associated illnesses in susceptible recipients of blood from seropositive donors. In a small number of patients followed serologically, an unusual pattern of EBV-specific antibodies has emerged. Prior to the disappearance of passively acquired IgG antibodies to viral capsid antigen (VCA) and to the EBV-associated nuclear antigen (EBNA), an upsurge in the anti-VCA titer occurs but little or no IgM anti-VCA develops which is contrary to expectations in a primary EBV infection. Several questions regarding transmission of EBV by blood transfusions remain unanswered. (1) How often does transmission occur? (2) What percentage of transfusion acquired infections are associated with clinical illness? (3) Is there a consistent characteristic antibody pattern in parenterally acquired infections which differs from that of natural infections? (4) What is the role of the level of passive antibodies and the volume of blood transfused in determining the establishment of an infection? This project will (1) identify the frequency of transmission of EBV by blood transfusion from antibody positive donors to seronegative recipients; (2) ascertain the risks of transfusion associated EBV infections in immunologically competent and immunocompromised children; and (3) answer the aforementioned questions about the degree of protection by passive antibodies and the serological response of the recipeint. Subjects will consist of children undergoing open heart surgery or bone marrow transplantation (BMT) and control groups of nontransfused patients hospitalized under similar circumstances. Serum and saliva will be obtained before the procedure and at about 2 weeks, 4 weeks, 12 weeks and 26-52 weeks afterwards. Sera will be titrate for antibodies to EBV specific antigens (VCA, EBNA, and early antigens) by indirect and anti-complement immunofluorescence and for virus neutralizing antibodies. The presence of EBV in saliva will be determined using the B lymphocyte transformation assay. Patients who develop serological and virological evidence of an EBV infection will be questioned and examined for clinical signs of illness. Patients undergoing BMT will be observed for the development of lympho-proliferative malignancies which arise at an enhanced frequency in renal transplant patients and were shown to be associated with EBV.