In 31 cases of acute nonlymphoblastic leukemia, bone marrow cells were serially cultured in semi-solid agar during the remission induction therapy. A normal in vitro cell growth pattern returned in 15 out of 22 patients up to 77 days before a complete remission was established by clinical and hematological criteria. In 6 cases the return of normal colonies coincided with clinical and hematological evidence of a complete remission. Nine patients failed to attain a remission and died from complications of bone marrow aplasia. Only one had a normal number of colonies and a normal cluster/colony ratio in cultures prepared 11 days after the completion of the first course of chemotherapy. At this time, his platelet count increased to normal level, possibly indicating a developing remission. Bone marrow cell culture criteria are useful in monitoring the remission induction therapy in patients with acute nonlymphoblastic leukemia. An early return of normal in vitro cell growth pattern suggests an approaching remission, which may be achieved several weeks later.