Sedimentation at unit gravity of human bone marrow cells, for 15 h at 4° C on a linear density gradient of Ficoll in culture medium ranging from 1.020 to 1.065 g/ml shows that a differential migration of the bone marrow cell sub‐populations exists with precise mean densities 1.021 ±lx 10−3 g/ml for lymphocytes; 1.024 ± 2.5 times 10−3 g/ml for non‐eosinophil granulocytes; 1.025 ± 2.5 times 10−3 g/ml for metamyelocytes; 1.030 ± 3.5 times 10−3 g/ml for other myeloid cells (myeloblasts, promyelocytes, myelocytes); 1.040 ± 3 times 10−3 g/ml for eosinophil granulocytes; and 1.055 ± 10 times 10−3 g/ml for megakaryocytes. The highest percentages of S phase cell and G2 and M phase cells determined by a cytofluorograph correspond to the peaks of immature myeloid cells (myeloblasts, promyelocytes and myelocytes). This method of bone marrow cell separation may be used to study the cell cycle in pathological bone marrows (leukaemia in particular) and to determine the effects and the efficiency of some antimitotics.