The main objectives of this research are: 1. To determine if the differences in response to therapy of children with ALL lie in the origin of the leukemic blasts (thymocytes vs. bone marrow stem cells) and to correlate cell origin with prognosis. Bone marrow and peripheral blood cells from children with untreated ALL will be studied to establish if a correlation exists between T and B cell-surface markers, presence of DNA terminal transferase in leukemic lymphoblasts, kinetics of colony formation by granulocyte precursors, clinical features and response to therapy. 2. To determine in children with ALL "in remission" the selective effects of four different antileukemia-drug combinations upon lymphocyte subpopulations and to establish whether a correlation exists between immunocompetence, as measured by "in vivo" and "in vitro" assays, and the clinical course of the disease.