Abstract Following a single injection of tritiated thymidine in dogs, a reproducible type of curve was obtained for the time sequence of the progression of labeled cells in separate mitotic phases, as observed on serial bone marrow aspirations. The duration of mitosis in erythroblasts at an advanced stage of maturation toward erythrocytes was shown to be of the order of 30 to 40 min for at least 50 per cent of the cells undergoing division at that stage. Mitotic duration did not appear to vary markedly for the remaining 50 per cent. The variability in the duration of the premitotic synthetic gap appeared more pronounced than the variability in the duration of mitosis. The incorporated tritiated thymidine did not of itself cause any appreciable change in these durations. The values obtained were shown not to be influenced by morphological criteria used to register cells in mitosis.