To clarify the mode of erythropoiesis, the in vitro proliferation of single human erythroblasts was recorded continuously for 80 hours by time-lapse, phase-contrast cinemicrography. Progenies from single erythroblasts were followed, their pedigrees were delineated, and their generation times were measured by counting the frames of the film. In one erythroblast pedigree, a daughter cell continued to divide three times to yield eight smaller erythroblasts; another daughter cell yielded four progenies, three of which abruptly lost most of their cytoplasm and became immobile. The generation time ranged from 15.8-30.0 hours (mean +/- SD: 23.3 +/- 4.8 hours), which corresponded to generation times calculated from in vivo data, such as the mitotic index, isotope labeling, or red cell turnover. Paired daughter cells showed very similar generation times. During succeeding mitoses, erythroblast size and nucleus/cytoplasma ratio decreased, cytoplasm darkened, and cell movement became more prominent. These studies on clonal cell proliferation using cinemicrography provide considerable information on the mechanism of hemopoiesis.