Quantitative in vitro tests showed that purified preparations of L-asparaginase from Escherichia coli were more toxic to blood lymphocytes from 12 of 15 patients with chronic lymphocytic leukemia than to lymphocytes from 25 persons with normal hemograms. Incubation for 7 days with 10 units per milliliter killed, on the average, 77 percent of leukemic and 34 percent of normal lymphocytes. The reagent produced appreciable toxicity to leukemic lymphocytes after 2 days of incubation.