Using immunogold staining of a suspension of living human epidermal cells to identify the Langerhans cell membrane-associated antigen T6 (revealed by the monoclonal antibody BL6), we have observed internalization of T6 antigen in Langerhans cells. This phenomenon is at least partly due to receptor-mediated endocytosis involving coated pits, coated vesicles, endosomes, the smooth endoplasmic reticulum, and lysosomes. These ultrastructural results suggest that T6 antigen may be part of a receptor site. Following receptor-mediated endocytosis, the appearance in the cell center of the first labeled Birbeck granules suggests that Birbeck granules could represent T6 intracellular transport organelles carrying T6 from the central part of the cell to an unknown destination.