The rate of 3H‐thymidine incorporation into DNA and labeling index were determined in fifteen adult patients with acute non‐lymphocytic leukemia by incubation of marrow aspirates in vitro with 3H‐thymidine before and 48 hours after receiving cytosine arabinoside. The cytosine arabinoside was administered using three different schedules, and was followed in 48 hours by high‐dose methotrexate plus Leucovorin and vincristine. The patients were treated with sequential courses of therapy, their clinical responses evaluated, and the results correlated with the rate of 3H‐thymidine incorporation into DNA and the labeling index obtained from marrow aspirates. Statistically significant differences in the initial DNA synthesis rate and increments thereof, and labeling index were found between those responding to chemotherapy and those not responding. High initial 3H‐thymidine incorporation and labeling index and the apparent ability to recruit additional cells into the growth fraction by chemotherapeutic manipulations seemed to define a population of leukemic patients susceptible to a cycle dependent regimen.

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