Dysregulation of neonatal myelopoiesis and thrombopoiesis predisposes the newborn to develop neutropenia and/or thrombocytopenia during states of increased demand. We have previously examined the effects of granulocyte colony-stimulating factor (G-CSF) alone or in combination with either stem cell factor (SCF) or interleukin-11 (IL-11) on in vivo neonatal rat hematopoiesis. In this study, we determined the effect of the triple combination of IL-11, SCF, and G-CSF on newborn rat hematopoiesis. Newborn Sprague-Dawley rats (< or = 24 hours old) were administered intraperitoneal (IP) injections of 250 micrograms/kg IL-11, 100 micrograms/kg SCF, 5 micrograms/kg G-CSF, or various combinations or phosphate-buffered saline (PBS)/human serum albumin (HSA) x 14 d. Platelet and blood cell counts were obtained on days 1, 3, 6, 8, 10, and 13; on day 14 bone marrow neutrophil storage pool (BM NSP), neutrophil proliferative pool (NPP), colony-forming units-granulocyte/macrophage (CFU-GM), and CFU-GM proliferative rates were determined. The triple combination failed to significantly increase the circulating hematocrit over other combinations or placebo. The circulating platelet counts, however, significantly increased during each of the IL-11 treatment arms, but they were not enhanced by the addition of either SCF, G-CSF, or the combination. The triple combination of IL-11, SCF, and G-CSF induced the most significant increase in the circulating absolute neutrophil count (ANC) above any other combination or placebo. Circulating ANC increased 12-fold following the triple combination vs. PBS/HSA (day 14 ANC 16525 +/- 1340 vs. 1368 +/- 197) (p < 0.001). The triple combination of IL-11, SCF, and G-CSF also induced the most significant increase in the BM/CFU-GM proliferative rate and BM NPP, p < 0.002 and p < 0.008, respectively. The highest increase in CFU-GM colony formation, however, occurred with both early lineage CSFs, that is, IL-11 plus SCF, and it was not further enhanced by the addition of G-CSF. These data suggest that the combination of two early-lineage CSFs, IL-11 plus SCF and G-CSF, significantly induces newborn rat myelopoiesis and that IL-11 alone significantly induces newborn rat thrombopoiesis. These results may be helpful in the design of future therapies to treat and/or prevent cytopenias in the newborn.