1. In short-term human bone marrow cultures, the generation times of three myelocytes were measured by phase contrast kinematography to be 29-30.5 hours. Two of these myelocyte generations were in direct succession of each other. 2. The mean mitotic time of 93 myelocytic mitoses was found to be 88 ± 20 minutes normally and 142 ± 13 minutes in 13 leukemic blast cells. This difference was statistically significant. 3. The maturation of myelocytic cells after their last mitosis from the metamyelocyte into mature granulocytes was followed and found to vary between 7 and 37 hours. This time difference was the result of the fact that one of the cells underwent a further division, where the three sister cells did not divide but matured only. All five final cells acquired the characteristics of band forms at about the same time. In eight other cells, the metamyelocyte-bandform transition time was found to be 7 to 30 hours. In all cells studied, the morphologic characteristics changed repeatedly between those considered typical for myelocytes and metamyelocytes. Thus, metamyelocytes were found to undergo division under the conditions of this study. 4. No evidence was found for amitotic divisions. However, the development of tetraploid cells was observed to be the result of endomitosis, of postmitotic formation of binucleated cells, or of formation of giant metamyelocytes by nuclear fusion of a binucleated cell.