Peripheral blood mononuclear cells (PBMC) and mobilized peripheral blood stem cells (PBSC) will be collected from patients who have leukemia or lymphoma or other medical conditions involving altered lymphohematopoietic stem cell or immunological function. These PBMC will be assessed for cellular markers by flow cytometry and will be studied for in vitro T helper, effector and suppressor cell functions, to determine whether cell mediated immunity is involved in the condition, or indicative of therapeutic efficacy or is predictive for outcome. PBSC will be assessed for cellular markers and in vitro growth characteristics that might be useful for identifying normal stem cell populations. Furthermore the capacity of stem cells to reconstitute the T lymphocyte lineage will be assessed using ex vivo explant cultures established with thymic or secondary lymphoid tissues.