The in vivo effects of purified human recombinant interleukin-6 (IL-6) on murine megakaryocytopoiesis were examined. IL-6 was administered subcutaneously to Swiss Webster mice, followed by evaluation of bone marrow megakaryocyte ploidy, size and frequency, and median platelet volume 24, 48, and 72 hours after the initiation of IL-6 administration. In addition, bone marrow megakaryocyte morphology was examined using electron microscopy at 72 hours. IL-6 (10,000 U per subcutaneous injection) was administered three times during the first 24 hours, three times during the second 24 hours, and twice during the last 24-hour period. IL-6 bioactivity (10 U/ng) was determined using the IL-6-dependent murine hybridoma cell line B9. Megakaryocyte ploidy distribution, measured by two-color flow cytometry, demonstrated a shift in the modal ploidy class from 16N to 32N and a significant increase in the relative frequency of 64N megakaryocytes 48 and 72 hours (but not 24 hours) after initiation of IL-6 administration (cumulative doses of 60,000 and 80,000 U at 48 and 72 hours, respectively). In addition, ploidy levels were increased in animals that received a cumulative IL-6 dose of only 40,000 U (evaluated after 72 hours). The size of recognizable bone marrow megakaryocytes, determined by the cross-sectional areas of plastic embedded bone marrow megakaryocytes, was increased at the 48-hour (60,000 U IL-6) and 72- hour (80,000 U IL-6) time points. Megakaryocyte frequency, measured by flow cytometry, was unaffected at all time points and doses of IL-6. Median platelet volume, measured by electrical impedance, was not consistently altered by administration of IL-6. Electron microscopic examination of bone marrow megakaryocytes showed an increase in the proportion of megakaryocytes with a wide, peripheral, organelle- deficient zone from 20% +/- 9% (SD) in control animals to 50% +/- 7% (SD) (P less than .02) in animals that received IL-6. No changes were observed in the distribution of the demarcation membranes. IL-6 is a potent stimulator of murine megakaryocytopoiesis, in vivo, and appears to act early in megakaryocyte differentiation.