ABSTRACT. Interferon (IFN)-α is a well known agent for treatment of viral and malignant diseases. It has several modes of actions, including direct influence on the immune system. We investigated IFN-α effects on peripheral blood mononuclear cells (PBMC) in terms of dendritic cell (DC) differentiation, as PBMC are exposed to high IFN-α levels during treatment of infections and cancers. We show that in vitro IFN-α exposure induced rapid and strong up-regulation of the DC-maturation markers CD80, CD86 and CD83 in bulk PBMC. Consistently, IFN-α induced up-regulation of these molecules on purified monocytes within 24 h. Up-regulation of CD80 and CD83 expression was IFN-α-concentration dependent. In contrast to GM-CSF+IL-4 generated DCs, most of the IFN-α-challenged CD83+ cells co-expressed the monocyte marker CD14. Despite a typical DC immunophenotype, cells never acquired a dendritic morphology. In mixed lymphocyte reactions IFN-α treated monocytes were significantly more potent than untreated monocytes to induce T-cell proliferation. At variance with untreated or GM-CSF+IL-4-exposed monocytes, IFN-α-challenged monocytes showed long-lasting STAT-1 phosphorylation. Remarkably CD83+CD14+ cells were present in varicella skin lesions in close contact with IFN-α-producing cells. The present findings suggest that IFN-α alone promptly generates non-dendritic antigen-presenting cells. This may represent an additional mode of action of IFN-α in vivo.