In vitro, IL-6 can induce hematopoietic progenitors to progress from Gointo cycle, but a role for IL-6 in regulating cycling status of progenitorsin vivohas not been established. In our studies, groups of five to six adult and newborn rats received i.v. injections of either IL-6 (1 ng/g body wt) or the vehicle (control), after which cycling of hematopoietic progenitors was evaluated by tritiated thymidine suicide. Progenitors from adult rats injected with the control had thymidine suicide rates of 7 \pm 1% (mean \pm SEM), compared with 23 \pm 7% in the IL-6 recipients (p< 0.02), Progenitors from newborn rats injected with the control had thymidine suicide rates of 19 \pm 2%, compared with 29 \pm 1% in the IL-6 recipients (p< 0.003). In addition, IL-6 administration resulted in release of cells from the nentrophil storage pool into the circulation, as evidenced by fewer polymorphonuclear cells flushed from the long bones (neonates,p< 0.001; adults,p< 0.003), a rise in blood neutrophil concentration (neonates,p< 0.001; adults,p< 0.005), and a leukocyte "left shift" (neonates,p< 0.001; adults,pPediatr Res28: 323–326,1990)