Approximately 18 years ago, the authors were able to produce an apparently successful bone marrow transplant by using umbilical cord blood. In view of the Chernobyl disaster and the subsequent problems of treatment with marrow transplantation, this study undertook to explore further the potential use of umbilical cord blood as a source of marrow cells. Specimens of umbilical cord blood were collected from 13 routine obstetrical deliveries. All specimens grew erythroid and granulocytic-monocytic colonies. The formation of these various hematopoietic colonies from umbilical cord blood was at least equivalent to bone marrow, and in some instances over 5 times more effective. There appeared to be a statistically significant correlation between the numbers of colony-forming units (CFU-E) and the male infants. The weight of the infants also showed a statistically significant correlation with the burst forming units, erythroid (BFU-E) and the granulocytic-monocytic colony (CFU-GM). The BFU-E also appeared to be greater in number when the time between collection and plating was shorter.