A class of primitive progenitor cells with high proliferative potential in vitro (HPP-CFC), has been identified in fetal liver and adult bone marrow both in murine and human systems. The kinetic properties of HPP-CFC2 and the more mature granulocyte-macrophage colony forming units (CFU-GM) derived from murine fetal liver on d13, d15 and d19 of gestation, newborn liver and neonatal liver on d3 and d8 postpartum have been evaluated and compared with the kinetic properties of these progenitor cell populations derived from adult bone marrow. The frequency of HPP-CFC2 in fetal liver was found to be greatest on d15 of gestation then subsequently declined in newborn and neonatal liver. Similarly, the highest proportion of HPP-CFC2 engaged in DNA synthesis (53±3%) was detected in d15 fetal liver. This proportion decreased to 13±2% in the liver 1 wk after birth, which is comparable to the number of HPP-CFC2 derived from adult BM which were in S-phase (10±1%). Production of CFU-GM was found to be greater in adult bone marrow than in either fetal or newborn liver. While the proportion of CFU-GM in S-phase was high in all 3 tissue samples, the greatest proportion of cycling CFU-GM (50±2%) was detected in d15 fetal liver. These results suggest that HPP-CFC2 derived from fetal liver are actively cycling while HPP-CFC2 derived from adult bone marrow are relatively quiescent. In contrast, a high proportion of CFU-GM derived from fetal, newborn liver and adult bone marrow are engaged in DNA synthesis.