The cellular composition of the human marrow was studied in thin sections of undecalcified bones of 24 fetuses aged from 11 to 22 weeks of gestation, embedded in methacrylate and stained by the Giemsa method. This procedure provides much better cellular detail than the decalcified bone sections embedded in paraffin used in previous studies. Hemopoiesis is already present at the 11th week of gestation in the long bones of the extremities. The cellularity of their marrow increases during the 4th month and is abundant in the months thereafter. Its composition is similar to that of adults but differs by the presence of a large number of stromal cells and by the absence of plasma cells and lymph follicles in the fetal marrow. Differential counts for blood-forming cells and stromal elements are given for the initial period of marrow development.