Albino guinea pigs with abraded skin were inoculated cutaneously with 27 strains of dermatophytes from animals and humans. The same strains were inoculated i.v. in guinea pigs with intact skin. Zoophilic dermatophytes and human isolates of Trichophyton mentagrophytes var. granulare produced ringworm after cutaneous application. After i.v. inoculation, the fungus was reisolated from skin samples from a considerable number of animals with and without clinical ringworm lesions, and also from lungs, liver, and kidneys. In the lungs, hyphal aggregates were noted. Some strains produced generalized dermatophytosis affecting all parts of the skin and internal organs. T. mentagrophytes B32663, selected for further study, was inoculated i.v. in the guinea pig, rabbit, rat, mouse, and chicken. Ringworm lesions occurred in the guinea pig and rabbit; in the mouse, rat, and chicken other organs were involved. Administration by other routes did not produce a generalized infection. The infection was not self-limited and the deep-seated lesions may be responsible for the recurrence of infection.