Thirty patients with vitiligo (ten of the segmental type and 20 of the generalized type) were sensitized with dinitrochlorobenzene (DNCB) in a normal skin site on the upper medial aspect of the arm. Challenge tests with dinitrochlorobenzene were performed in vitiliginous patches and in normal skin sites. In vitiliginous patches diminished contact sensitivity reactions to dinitrochlorobenzene were noted in both patient groups, while in normal skin sites a normal delayed hypersensitivity response to the same antigen developed in the same patients. Tuberculin reactivity was not suppressed in vitiliginous lesions. We suggest that diminished contact reactivity in vitiliginous skin might be due to functional changes in Langerhans' cells, or to an alteration of carrier (skin) proteins in the lesions.