In response to the need for better quantitative estimates of the regional distribution of the active bone marrow organ in infants and children, a method using various anatomical data has been developed. The method, a refinement of that of Atkinson (1962), predicts that in the newborn 27.8% of the active marrow in the body resides in the skull and 20.7% resides in the lower limbs. Atkinson's method predicts 7.0% in the skull and 38.9% in the lower limbs. According to the experimental data of Hudson (1965) involving 16 late-term foetuses and newborns, there is 29.5% (+/- 4.2%) in the skull and 23.7% (+/- 2.2%) in the lower limbs. The values from the new method are much closer to experimental values in both bone groups. The values for the newborn and age one year predicted by the new method were adjusted after comparison with the experimental data for the newborn. Newer information on marrow cellularity was also incorporated into the method presented here, so that the distribution calculated here for the adult differs somewhat from those calculated by others. Overall, this adult distribution agrees more closely with published distributions based on 59 Fe studies than do the earlier distributions.