Some properties of hemopoietic stem cells (CFUc) in human cord blood and peripheral blood of neonates were compared with those in bone marrow, by the method for in vitro growth of colony-forming cells in semisolid agar. The number of CFUc circulating in cord blood was 93.5 +/- 57.8 (mean +/- 1 SD)/10(5) cells, which was slightly less than that of bone marrow. The number of CFUc in peripheral blood decreased promptly following birth. From cord and neonatal blood, most colonies were monocyte/macrophage colonies (60.0 +/- 4.6%), whereas those from bone marrow were largely granulocyte colonies (88.3 +/- 6.2%). The differences of growth curve, colony size, and turnover state of CFUc between cord blood and bone marrow seemed, in part, to result from the dominancy of monocyte/macrophage colony-forming cells in cord blood. The cell cycling rate of CFUc in cord blood was remarkably lower than that in bone marrow. From these results, it seems clear that CFUc in cord blood differ in some aspects from those in bone marrow. However, the precise physiologic and functional significance of CFUc circulating in cord and neonatal blood remains unknown.