Nine adults with acute myeloblastic leukemia were treated with L-asparaginase, a new anti-neoplastic agent. One of these patients achieved a complete remission. This case is presented in detail including serial determinations of serum L-asparaginase levels during the treatment. The mechanism of action, toxicology and the present status of the effectiveness of L-asparaginase in acute leukemia are presented. Optimal dose schedule and possible future role of L-asparaginase in the treatment of leukemia is discussed.