AbstractLymphokine‐activated killer (LAK) activity was studied in 29 children with acute lymphoblastic leukemia (ALL). Peripheral blood lymphocytes of the patients and normal volunteers were cultured in the presence of recombinant‐interleukin 2 (R‐IL2) for five days, and assayed for their LAK activities by51Cr release assay using Raji cells as a target. At the time of onset, LAK activity was 4.3±2.6% (mean±SD, n=6) compared to the normal value. During remission LAK activity improved to 49.6±30.2% (n=16, p<0.01). After cessation of therapy LAK activity further improved to 97.3±21.2% (n=7, p<0.01). The conventional anti‐leukemic agents appeared to be suppressive for LAK activityin vitro.Of the patients in remission for more than one year, three with depressed LAK activity relapsed one to four months after the assay. Although it still remains undetermined whether the depression of LAK activity is responsible for the subsequent relapse, the application of adoptive immunotherapy using LAK and R‐IL2 is of clinical interest because it can be expected to enhance the activity of LAK. As compared with natural killer cell activity in childhood ALL, LAK activity showed a prompt improvement af