The NHLBI Stem Cell Allotransplantation Program is researching methods to improve immunity against infectious organisms following allogeneic stem cell transplantation (SCT). We are particularly interested in cytomegalovirus (CMV), which can cause serious problems in patients following transplantation. In the future, we hope to design ways to improve immune responses to CMV in transplant recipients, such as vaccinating transplant donors and/or patients against this virus. In order to characterize the effect of any such intervention on CMV immunity, we first need to better understand CMV immune responses in normal, healthy persons. This involves designing and validating an in vitro assay, which can reliably and consistently detect immune responses against CMV. Furthermore, we hope to define the temporal variability that exists in CMV responses in healthy individuals, which is essential for the design and implementation of all future studies. We plan to collect blood samples from 20 normal volunteers who have been previously exposed to CMV (CMV seropositive individuals). Eligible individuals will be asked to donate blood on five occasions, once a week for four weeks and a fifth time, two months later. Eligible individuals must be 18-65 years of age, otherwise healthy, and seropositive for CMV exposure.