Marrow specimens obtained from 23 patients with acute myelocytic leukemia were exposed to cytosine arabinoside and/or daunorubicin in vitro, and the effects of these agents on colony formation in vitro was determined. Thymidine suicide indices were determined as well, which permitted a distinction to be made between kinetic and metabolic resistance to cytosine arabinoside. The sensitivity of the colony- forming cells to the two chemotherapeutic agents did not correlate with each other, indicating that sensitivity to each was independently determined. The relationship between in vitro sensitivity to daunorubicin and cytosine arabinoside and response to 25 courses of in vivo therapy with these two agents administered to 21 patients was determined. These studies indicated a clear-cut relationship between in vitro drug sensitivity and in vivo response with patients whose leukemic cells were sensitive to both agents entering complete remission, whereas patients whose leukemic cells were insensitive to one or both drugs in vitro failed to enter remission.