We have purified and further characterized a histamine releasing factor (HRF) derived from human mononuclear cells using gel-filtration HPLC, reverse-phase HPLC, anion exchange chromatography, and elution from SDS gels after electrophoresis. Considerable heterogeneity is seen, far exceeding that published in prior reports. Gel filtration HPLC yielded a major peak at molecular weight 30,000 and minor peaks at 50,000 and 12,000. Reverse-phase HPLC gave one major fraction in the void volume and an eluted peak at 50-60% acetonitrile. Accell QMA anion exchange HPLC revealed three peaks of activity; one in the void volume similar to that published previously using QAE-Sephadex, and peaks that eluted at 0.5 and 0.8 M ammonium acetate, respectively. Electroelution following SDS-PAGE yielded peaks at MW 12,000 and 15-17,000 plus variable peaks at 25-27,000, 31-34,000, and 80-90,000 D. Using a combination of the aforementioned procedures, we have purified molecular species of HRF at 41,000 and 17,000 D to apparent homogeneity, as judged by SDS PAGE and autoradiography. Since human interleukin 3 and granulocyte-macrophage colony-stimulating factor possess histamine-releasing capability, it is clear that multiple cytokines can share this activity. However, the major HRF we isolate from human mononuclear cells appears, thus far, to be unique.