To investigate the proliferative characteristics of human bone marrow leukemic cells in vivo, we developed a new method using bromodeoxyuridine (BrdU) and its monoclonal antibody. Thirty minutes after a bolus BrdU infusion, a 1st bone marrow aspiration (BMP) was performed to calculate the BrdU labeling index (BrdU-LI). Then, a 150-min continuous infusion of BrdU was started. Immediately after the infusion, a 2nd BMP was done to analyze the increasing rate of the BrdU-LI for 3 h. Each smear was stained by an immunohistochemical method. The BrdU-LI, the duration of S phase (Ts) and the cell cycle time (Tc) of leukemic cells from 13 previously untreated patients with acute leukemias were estimated. A mean value of 7.2% for BrdU-LI was obtained. Furthermore, mean values of 9.7 h and 152.5 h for Ts and Tc respectively were calculated. This method may prove to be useful in the in vivo cell cycle analysis of leukemias and other malignancies.