A heteroantiserum, prepared in rabbits against fractionated cell membranes of a human B-lymphoblastoid cell line, was used to study the distribution of Ia antigen(s) in human epidermis. Indirect immunofluorescence staining demonstrated specific reactivity of dendritic supra-basal cells, consistent in location with Langerhans cells. Basally located cells were noted in biopsy specimens from vitiliginous skin and from the leukodermatous regions of halo naevi. The specificity of the reaction was confirmed at the ultrastructural level by means of ferritin labelling methods. Cell surface staining was confined in the epidermis to Langerhans cells. Fc and C3' receptors were studied by means of rosetting methods. Negative results were obtained on frozen sections, while 2-3% of cells formed rosettes when applied to an epidermal cell suspension.