Circulating CFU-GM, BFU-E and CFU-MIX were assayed in 21 infants aged between 1 day and 44 weeks. Compared to 15 adults, progenitor cells of all types were increased until 10 weeks following birth and approached the respective ranges of adults thereafter. The mean increases of progenitor cells in infants aged between 1 day and 10 weeks were 26-fold for CFU-GM, 7-fold for BFU-E and 5-fold for CFU-MIX. Our results demonstrate that not only committed progenitor cells (CFU-GM, BFU-E), but also early progenitor cells with the capacity for self-renewal (CFU-MIX), are increased in early infancy. These data further support the hypothesis that high levels of blood progenitor cells in very early stages of life reflect the colonization process of developing bone marrow by circulating progenitor cells and demonstrate the terminal phase of this process during the first 10 weeks after birth.