A technique to quantify the cytoreduction in the bone marrow induced by chemotherapy in leukemic patients is reported. Bone-marrow core biopsies are obtained and the specimens are carefully minced in McCoy's 5a medium with a 20% fetal bovine serum supplement. The number of cells present per millimeter length of bone-marrow specimen is obtained by finding the concentration of cells in the supernatant, multiplying by the total volume of the supernatant, and dividing by the length of the bone-marrow biopsy specimen. Cytocentrifuge preparations are made from this supernatant and stained by standard techniques. Once the percentage of blasts is found, the absolute number of blasts per millimeter of bone marrow can be calculated. Serial measurements were made in 19 patients: nine patients were receiving first-line therapy for acute nonlymphocytic leukemia, and 10 were given experimental chemotherapy, which consisted of prolonged infusions of thymidine and cytosine arabinoside in six, and short infusions of thymidine, cystosine arabinoside, and phosphono-acetyl-l-aspartate, daily for five days in four. No patient with less than 2 log(10) reduction in the number of marrow blasts achieved a remission, while only one patient in whom there was more than a 2 log(10) reduction in the number of blasts failed to do so.