The central hypothesis to be tested by the clinical biochemical studies proposed in this application is that the combination of methotrexate (MTX) and 1-B-D-arabinofuranosycytosine (araC), when appropriately scheduled, has synergistic therapeutic activity in the treatment of leukemias. This overall objective is divided into two specific aims. The first aim is to examine the biochemical interactions of MTX and araC in acute nonlymphocytic leukemia in relapse. The second aim is to test the hypothesis that MTX pretreatment can increase the intracellular activation of araC in the hematopoietic cells of children with acute lymphocytic leukemia in remission. The intent is to correlate the biochemical findings with the clinical course of the disease. The principal methods to be used include the measurement of intracellular nucleotides by high-performance liquid chromatography (hplc) and enzymatic analysis; the measurement of plasma drug levels in patients by hplc; the quantitation of free intracellular MTX by a competitive-binding assay; and the measurement of bioactive plasma folates by microbiological assay. This project will contribute to understanding the biochemical modulation of the intracellular metabolism of araC by MTX and will establish whether or not the biochemical data correlate with clinical response.