Bone marrow findings after treatment with recombinant human interleukin-3. In a phase I/II study, bone marrow biopsy specimens and aspirates of 20 patients with malignant tumors but normal bone marrow (n = 6), bone marrow failure resulting from chemotherapy (n = 4), myelodysplastic syndrome (n = 5), and aplastic anemia (n = 5) were evaluated before and after patients were treated with recombinant human interleukin-3 (rhIL-3). This cytokine proved to be an effective hematopoietic growth factor with only mild side effects. The rhIL-3 treatment led to increased overall bone marrow cellularity with trilinear stimulation of hematopoietic cells, except in most patients with aplastic anemia. In all patients, significant eosinophilia and, in some instances, bone marrow fibrosis developed. In addition to the increase in the number of circulating neutrophilic granulocytes, platelets, and reticulocytes, an increase of peripheral blood monocytes and lymphocytes was observed. The histologic and cytologic findings support the concept that rhIL-3 stimulates the proliferation and differentiation of pluripotent hematopoietic progenitor cells. It appears to be a safe and efficient therapeutic modality in patients with bone marrow failure. Additional clinical studies are needed to determine which patients will profit most from rhIL-3 treatment. 