A new technique is introduced for determining the number of bone marrow cells per cubic millimeter marrow, providing an accurate and objective means for quantitating therapy-induced cytoreduction. The method requires a correction for admixed peripheral blood in bone marrow aspirates to measure the fraction of remaining pure marrow. While cell kinetic differences between blood, aspirates, and biopsies identify the proportion of contaminating blood cells, the ratio of red cell hematocrits in blood and aspirate gives the volume of trapped blood. By combining both procedures, bone marrow cell counts per unit volume pure marrow result (BMC/cu mm BM), which were found highly reproducible. Blast cell counts (BMBC/cu mm BM) were obtained by additional morphological differentiation. BMC and BMBC/cu mm BM were monitored in 16 patients with acute nonlymphoblastic leukemia treated with daunorubicin, cytosine arabinoside, and 6-thioguanine in combination and in 4 patients with end-stage acute leukemias and non-Hodgkin's lymphomas during high-dose thymidine therapy. Total and daily therapy- induced cytoreduction rates were significantly greater (P less than 0.01) in responders than nonresponders to either regimen. Changes in BMC/cu mm BM were also found representative for changes in BMBC/cu mm BM, since the majority of bone marrow cells were blasts. In acute leukemia. BMC/cu mm BM thus provides accurate and objective measurements of treatment efficacy in vivo and after short periods of drug exposure. Differences in cytoreduction rates within the group of responders also suggest possible prognostic implications.