Granulocytic-monocytic colony-forming activity of marrow cells was evaluated at presentation in 20 patients with acute nonlymphocytic leukemia (ANLL). A double-layer agar culture was employed to determine colony- and cluster-forming activity. All patients underwent induction chemotherapy with cytosine arabinoside and daunorubicin. Five patients died during induction. Eight of the 11 patients entering complete remission had absent or decreased colony and cluster growth of marrow cells. Four patients who failed to obtain a complete remission produced increased or only cluster growth in culture. Our data support the conclusions of others that decreased colony and cluster formation in vitro is associated with a more favorable response to induction chemotherapy for ANLL.