Enzyme therapy directed at depletion of an amino acid, asparagine, indispensable for tumor cell growth but not required by normal tissues, offers a new and promising approach to the problem of treatment of leukemia and disseminated cancer. Therapy with the enzyme L-asparaginase in three cases of acute lymphatic leukemia resulted in measurable improvement in two advanced cases. In the third case, there was a striking response to large dosage, with rapid necrosis of lymph node masses and clearing of leukemic infiltrates. A laboratory test to detect dependency of tumor cells on asparagine seems helpful for selection of cases for investigation of therapy. Furthermore, a variety of human malignant diseases including acute lymphatic leukemia and acute and chronic granulocytic leukemia have been shown to be asparagine-dependent by this test. Exploitation of further possible nutritional differences offers a fertile field for investigation.