Cell-mediated immune responses were measured in 91 patients with dermatophytosis by means of delayed-type skin hypersensitivity to a purified trichophytin preparation (ethylene glycol method) and to tuberculin (purified protein derivative, PPD). The findings indicate that dermatophytes differ in their sensitizing capacity as measured by trichophytin skin sensitivity. Trichophyton mentagrophytes appeared to be a potent sensitizer compared with Trichophyton rubrum (p < 0.01), whereas Epidermophyton floccosum appeared as a moderate sensitizer. The localization of infection also affected the cell-mediated response to trichophytin, i.e. the frequency of reactions was 44% in tinea cruris and 33% in tinea pedis, while tinea pedis with nail infections elicited delayed-type reactivity in 60% of cases. On the basis of the significant difference (p < 0.001) in cell-mediated reactivity between chronically and non-chronically infected subjects as measured with trichophytin, it is concluded that the cell-mediated response is of importance for the development of host resistance to dermatophytic infections. This study provides further evidenced in support of the view that a partial defect in the cell-mediated system may be responsible for establishment of chronic dermatophytosis.