Experimental dermatophyte (Trichophyton mentagrophytes) infections were initiated by using quantified spore inoculi on the forearms of 13 healthy volunteers who manifested no evidence of prior infection. Active lesions typical of tinea corporis developed. Following the emergence of delayed-type hypersensitivity, the lesions resolved spontaneously. Subsequently, seven of the now immunologically experienced volunteers were reinoculated. Early, atypical inflammatory lesions developed. There was no spread of infection, the organism rapidly disappeared, and the course of infection was shortened. The number of spores required to initiate an infection that persisted three weeks increased from less than six to approximately 300. Thus, the acquired immunity was relative. Cell-mediated immunity correlated with this enhanced resistance. Cell-mediated immunity is hypothesized to act indirectly by destroying the "privileged" nature of the stratum corneum.