SummaryBacterial sepsis is a relatively common problem in the neonatal period, particularly among prematurely delivered infants. The newborn rat has been widely used as a model for sepsis neonatorum, and in that model incomplete development of the neutrophil system has been postulated to be an important factor predisposing neonates to death from bacterial infection. In this study, that hypothesis was further tested by assessing neutrophil development in rats of various pre- and postnatal ages. Using standard soft agar colony techniques for detecting granulocyte-macrophage progenitor cells [CFU(c)], the number of CFU(c)/g of body weight was seen to increase from 0.5 + 0.1 X 103at 19-20 days gestation to 10.5 ± 0.2 x 103at 4 weeks. The anatomic location of CFU(c) changed from totally hepatic at 16 days gestation to almost totally myeloid at 4 weeks. Lastly, the proportion of mature, stored neutrophils/CFU(c) decreased from 2440 ± 40 at 19-20 days gestation to 430 ± 75 at 4 weeks.