To prove the existence of hematopoietic stem cells in human cord blood, two techniques were employed: 1) colony formation in methylcellulose cultures; 2) the diffusion chamber technique, in which semipermeable chambers containing cell suspensions are implanted into the abdominal cavity of irradiated mice. Starting solutions were highly purified lymphoid cell suspensions.In methylcellulose cultures from cord blood the mean number of colonies per plate is 60. In cultures from adult blood there are 0 to 8 colonies. Infants 8 to 10 days old have intermediat values. In diffusion chambers filled with cord blood, both, myelo- and thrombopoiesis is much more active than in chambers with adult blood. In cord blood, myelopoiesis is evident from the 6th day after implantation and megakaryocytes appear arounc the 10th day.The results show that in human cord blood there are hematopoietic stem cells among the lymphoid cell population: 0.06 percent myelopoietic stem cells and, in addition, thrombopoietic stem cells.