For the inhibition of dihydrofolate reductase by methotrexate, the rate constants, dissociation constants of various enzyme-substrate and enzyme-inhibitor complexes, and the mechanism of inhibition (i.e. competitive or non-competitive) will be determined. The effects of 6-azauridine on the nucleotide concentrations of the mouse leukemia cells (L5178Y) cultured in vitro will be determined by the technique of high pressure liquid chromatography. Explanations will be sought for the lack of orotidylate accumulation and the temporary increase of adenosine triphosphate concentration after the treatment with 6-azauridine. BIBLIOGRAPHIC REFERENCES: Tight-Binding Inhibitors: I. Kinetic Behavior. Cha, S., Biochemical Pharmacol. 24: 2177-2185 (1975). Tight-Binding Inhibitors: II. Non-steady State Nature of the Inhibition of Milk Xanthine Oxidase by Allopurinol and Alloxanthine and of Human Erythrocytic Adenosine Deaminase by Coformycin. Cha, S., Agarwal, R.P. and Parks, R.E., Jr., Biochem. Pharmacol. 24: 2187-2197 (1975).