The cellular composition of the granulocytic series in normal human bone marrow was studied in aspirates of different volume. An increase in the proportion of polymorphonuclears was observed in samples of more than 1 ml, thus indicating a dilution of bone marrow by peripheral blood. Certain data from the literature, referring to the differential count in the bone marrow granulocytic series, are discussed. The differences between these data can be explained by the dilution of bone marrow cells by mature granulocytes in blood.