Langerhans cells and thymocytes were comparatively studied for their monoclonal antibody-induced endocytic activity. The two cell types were CD1a or Class I-HLA immunolabeled, incubated under similar conditions and the effects induced on membrane mobility were analyzed by a fluorescence method for the thymocytes and at the ultrastructural level for both cell types. We provide evidence that thymocytes are able to cap and internalize by receptor-mediated endocytosis CD1a antigen and Class I-HLA, whereas Langerhans cells present only a process of internalization by receptor-mediated endocytosis for both membrane antigens.