Human epidermal Langerhans cells (LC) isolated from normal skin were infected in vitro with human immunodeficiency virus type 1 (HIV1). To control the permissivity of LC for HIV1, cells isolated from the epidermal sheet of normal skin by trypsinization were cocultured with HIV1-carrying promonocytic cells (U937) and observed by electron microscopy. An early sign of infection occurring in the coculture was the formation of retroviral type buds from LC membrane. Different steps in the process of viral budding up to virus release into the extracellular space were observed by electron microscopy. Treatment with either coupled phorbol esters/bacterial lipopolysaccharide or a recombinant cytokine (tumour necrosis factor alpha) did not significantly enhance viral production. The ability of in vitro infected LC to transmit virus to other haematopoietic cells and the consequences of such an infection on antigen-presenting function of LC remain to be elucidated.