The purpose of this core is to provide a centralized laboratory resource for cryopreservation and distribution of patient samples and to monitor the kinetics of immunologic reconstitution following allogeneic hematopoietic stem cell transplantation (HSCT). Prior to transplantation, peripheral blood mononuclear cells (PBMC), bone marrow and serum or plasma are obtained from all patients enrolled on clinical trials supported by this Program Project. PBMC and plasma or serum are also obtained from normal stem cell donors for these individuals. Following HSCT, PBMC, DNA and plasma samples are cryopreserved at regular intervals. These samples are made available to all of the investigators in this program. Immune reconstitution after HSCT is evaluated through a variety of methods that provide a quantitative assessment of specific lymphocyte populations as well as their level of maturation and function. Quantitative analysis of circulating lymphocytes is determined by multi-parameter flow cytometry with a panel of fluorochrome-conjugated monoclonal antibodies. This is primarily evaluated using fresh samples and semi-automated methods for immunofluorescence analysis of whole blood. Assays of immune function primarily utilize cryopreserved PBMC. Reconstitution of T cell receptor repertoire is examined by the method of TCR Vp spectratyping. Thymic function is evaluated by quantitative PCR measurement of T-cell receptor excision circles (TREC) in PBMC. Reconstitution of T cell immunity to specific target antigens such as CMV and EBV is determined by ELISPOT. Results of each of these assays can be correlated with other parameters of immune function as well as with clinical outcomes. The availability of pre-HSCT samples as well as normal donor samples provides additional control populations and will allow additional comparisons. This core has 3 Specific Aims: 1. To obtain, isolate and cryopreserve mononuclear cells, DNA, RNA, plasma and serum components from patient samples for subsequent study by program investigators. 2. To maintain a computerized inventory of banked samples, assure patient privacy and distribute samples in a timely and equitable fashion to laboratory investigators for experiments proposed in this application. 3. To provide phenotypic and functional measurements of immune reconstitution in peripheral blood, bone marrow and targeted skin biopsies following allogeneic HSCT.