Epidermal Langerhans cells bear surface receptors which implicate them as immunocompetent cells and they are now felt to play an important role both in delayed hypersensitivity and in skin allograft reactions. To determine the relationship between Langerhans cell availability and certain immunologic phenomena, surface densities were determined by ATP-ase and gold uptake in 3 rodent species: guinea pig, hamster, and mouse. Surface densities in epidermal specimens from the ear, back, foot pad, and buccal mucosa varied between 600 and 1500 cells/mm². Significantly fewer cells were found in the hamster cheek pouch (130 cells/mm²) and in the mouse tail (110 cells/mm² for C57BL/6J; 260 cells/mm² for BALB/c nu/nu). Langerhans cells were absent from the central portion of the cornea in all 3 species. Decreased Langerhans cell surface density may contribute to immunologic privilege as has been observed for the cornea and hamster cheek pouch and to the unusual allograft characteristics of mouse tail skin.