Plasma concentrations of IFN‐α are increased in several inflammatory conditions. Several lines of evidence indicate that IFN‐α has anti‐inflammatory properties. To study the effects of IFN‐α on leucocyte subsets and activation and on cytokines, we administered IFN‐α (rhIFN‐α2b; 5 × 10⁶ U/m²) to eight healthy human subjects in a randomized controlled cross‐over study and analysed changes in circulating leucocytes and parameters for neutrophil and monocyte activation. After administration of IFN‐α, neutrophil counts increased, monocyte counts decreased transiently, whereas the number of lymphocytes, basophils and eosinophils showed a sustained decrease. IFN‐α administration was also associated with neutrophil activation, reflected in an increase in the plasma concentrations of elastase–α₁‐antitrypsin complexes and lactoferrin. Serum neopterin, a marker for monocyte activation, was significantly increased 10 h after administration of IFN‐α. IFN‐α significantly increased plasma concentrations of IL‐6, IL‐8 and IL‐10. Although IL‐1 and tumour necrosis factor (TNF) remained undetectable, plasma concentrations of soluble TNF receptors p55 and p75 increased after IFN‐α administration. We conclude that IFN‐α induces multiple alterations in the distribution and functional properties of leucocytes. IFN‐α exerts pro‐ as well as anti‐inflammatory effects within the cytokine network.