In a phase I/II study, 11 patients with marrow failure (10 with acquired aplastic anaemia and one with pancytopenic Fanconi anaemia) were treated with recombinant human interleukin-6 (rhIL-6) to assess the safety and tolerability of rhIL-6 and its effects on peripheral blood counts, bleeding complications and transfusion requirements. All patients with acquired aplastic anaemia were refractory to immunosuppressive treatment or had relapsed after immunosuppressive therapy and were not bone marrow transplantation candidates. Recombinant hIL-6 was to be given as a once-daily subcutaneous injection for 28 d at doses ranging from 0.5 to 5.0 micrograms/kg. After an observation period of 2 weeks, five patients received a second treatment course of 28 d. Only one patient had a sustained increase in platelet count from 18,000 to 72,000/microliters. Bleeding occurred in four patients and caused premature discontinuation of rhIL-6 therapy in three patients. A deterioration of pre-existing anaemia was observed in nine patients. No significant changes of leucocyte counts were observed during the first cycle. During the second cycle the peripheral blood monocyte counts decreased significantly. No significant changes in bone marrow cellularity were observed. Recombinant hIL-6 induced a dose-dependent increase in acute-phase reactants in all patients. Other adverse events included fever, headache, arthralgia, tachycardia and hypertension. In conclusion, rhIL-6 given alone at low doses does not increase platelet counts in the majority of patients with aplastic anaemia and can precipitate a sudden worsening of pre-existing anaemia and thrombocytopenia. This study was discontinued prematurely on account of the toxicity of rhIL-6 seen in patients with aplastic anaemia.