The CFU-Gm content of liver, spleen and bone marrow of human fetuses was determined from the 8th to 28th week of gestational age. The study of progenitor cells from 43 fetal livers revealed that the CFU-Gm increased from 8 to 12 weeks, was relatively stable through 16 weeks and thereafter declined by 22 to 24 weeks. The total number of liver CFU-Gm rose from 10(4) to greater than 10(6) CFU-Gm by the 16th week of gestation and declined to approximately 4 X 10(5) progenitor cells by 22 to 24 weeks. Peak values of bone marrow CFU-Gm were noted at 10 to 14 weeks of gestation. Generally, the numbers of these progenitor cells declined by 16 to 18 weeks; however, several high values were obtained at later times. Splenic CFU-Gm were relatively constant in number between 12 and 28 weeks of gestation with no correlation with fetal age. Thus, these studies indicate that certain fetal tissues contain substantial numbers of hemopoietic progenitor cells. Based on the quantity of CFU-Gm obtained, it appears feasible to use fetal liver cells as a source of progenitor cells for transplantation.