Langerhans' cells (LC) are epidermal bone-marrow-derived dendritic cells. They represent in mankind 1 to 6% of the epidermal cells from which they can be distinguished by specific phenotype (membrane receptors and antigens related to the immune function) and by ultrastructural specific organelles: the Birbeck granules. In dogs and cats, such cells were recently described; they display a phenotype very similar to that of human LC (CD1, CD8, CD11/18, CD45 and MHC II positive for canine LC, and CD18, CD4, panleukocyte antigen and MHC II positive for feline ones) and in both species, Birbeck granules are observed. Furthermore occurs in dog a benign self-healing LC tumor: the canine cutaneous histiocytoma (CCH). This tumor exhibits numerous comparison points with a human LC disorder named Hashimoto-Pritzker disease, and thus may constitute an interesting model to explore causes of such a proliferation and mechanisms of tumor rejection. In 1986, Pedersen isolated in cats a new retrovirus very similar to the human immunodeficiency virus (HIV), the feline immunodeficiency virus (FIV). Since human LC may be infected by HIV, feline LC may represent a good candidate for an FIV model for exploring the infection of human LC by the HIV and for shedding light on the role of human LC located in the mucous membranes in the initial viral inoculation process.