Human umbilical cord blood cells from 26 newborn infants and peripheral blood cells from 18 adults were cultured in vitro by using the agar-gel method of human hemopoietic cell culture. An increased concentration of colony-forming cells was seen in the cord blood cultures. Between 17 and 385 colonies, with a mean of 122, were formed in these cultures per 2 x 105 nucleated cells plated. The peripheral blood cell cultures from adults gave rise to 0-11 colonies, with a mean of 3, per 2 x 105 nucleated cells plated. The average number of cells per colony was 1000-1500 cells after 14 days of culture, predominantly granulocytic.