Interleukin 6 (IL-6) has been demonstrated to possess a variety of biological and immunological functional properties. Recently, Interleukin 6 has been shown to stimulate in vitro fetal hematopoiesis. In the present study, we investigated the effects of 7 days of rhIL-6 on newborn rat in vivo myelopoiesis. Sprague-Dawley newborn rats (less than 24 h old) were injected (intraperitoneally) daily for 7 days with either rhIL-6 or PBS/BSA. RhIL-6 induced a mild but significant increase in the peripheral neutrophil count on day 1 (P less than 0.02), but had no significant sustained effects during the remaining 7 days of administration. Additionally, rhIL-6 had no significant effect on the bone marrow neutrophil proliferative pool, neutrophil storage pool, the liver/spleen neutrophil storage pool and neutrophil proliferative pools. Although rhIL-6 induced a significant increase in the day 1 platelet count (p less than 0.03), it failed to induce a sustained significant increase in the platelet count during the remaining 7 days of administration. RhIL-6 also failed to induce any change in the circulating hematocrit during the 7 days of administration. This study suggests that rhIL-6 failed to induce a significant and sustained increase in the peripheral neutrophil count or platelet count during in vivo administration in neonatal rats. Although rhIL-6 may possess additional immunomodulating effects in the newborn, this 7-day study of rhIL-6 failed to demonstrate any induction of neonatal peripheral neutrophilia or modulation of neonatal myeloid proliferation.