We examined the effects of recombinant human interleukin 11 (rhIL-11) on in vivo human hematopoiesis. Twelve women with advanced breast cancer and no evidence of bone marrow (BM) involvement were treated with 10, 25, 50, or 75 micrograms/kg/day of rhIL-11 administered subcutaneously for 14 consecutive days. Examination of bone marrow trephine biopsies obtained before and after rhIL-11 treatment revealed unchanged BM cellularity at all doses, and a statistically significant increase in megakaryocyte (MK) frequencies (from 0.5+/-0.1% to 1.0+/-0.3%) following administration of the two highest doses (p< 0.001). The BM biopsies also showed an increased proportion of immature myeloid and erythroid precursors following 14 days of treatment in all cases. The mean proportion of marrow cells stained with PC10, a monoclonal antibody (mAb) that recognizes the proliferating cell nuclear antigen (PCNA), increased from 16.3+/-5.7% to 45.8+/-17.1%(p< 0.001) following the two highest treatment doses. Most of the PC10+ cells were promyelocytes and proerythroblasts. In this same group, the proportion of PC10+ MKs increased from 28.3+/-11.5% to 56.8+/-24.3%(p< 0.01) after treatment, while megakaryocyte ploidy analysis revealed a greater number of higher ploidy (64N) megakaryocytes following rhIL-11 treatment (p< 0.012). Numbers of BM and peripheral blood (PB) CD34+, CD34+ DR+, and CD34+ DR-cells did not change following rhIL-11 treatment. Following rhIL-11 therapy at the highest dose studied, a 3-and 10-fold increase in the number of committed BM MK progenitor cells (CFU-MK) was observed in two of three patients, while no change was seen in the number of the other BM or PB progenitor cells examined. rhIL-11 administration was also associated with an increase in BM reticulin content (fibrosis grade 1-2) in 7 patients. These results indicate that the administration of rhIL-11 to patients with normal hematopoiesis stimulates MK endoreduplication, PCNA expression, and, at high doses, increases MK and CFU-MK progenitor cell numbers. In addition, rhIL-11 was able to stimulate precursor cells of different marrow lineages without affecting the number of assayable progenitor cells.