Peripheral blood mononuclear cells (PBMC) 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. Peripheral blood stem cells (PBSC) from untreated donors and from cytokine treated donors will be assessed for cellular markers and in vitro growth characteristics that might be useful for identifying normal stem cell populations.