The development of cellular hypersensitivity, as measured by lymphocyte blastogenic and skin tests assays, was correlated with intensification of lesion erythema in guinea-pigs infected with Trichophyton mentagrophytes. Development of lymphocyte blastogenic responses to trichophytin was assayed using forty infected animals and twenty controls; blastogenic conversion occurred between 9-11 days post spore application. The onset of cellular hypersensitivity was assayed in vivo in twenty infected guinea-pigs by skin testing at intervals after infection; positive responses occurred between 9-11 days post spore application which coincided with the time of blastogenic conversion. Clinical characteristics of lesions were followed in a group of twenty-four animals: lesions first appeared on day 7; erythema intensified until day 10; lesions reached maximum size by day 13; and alopecia began on day 17. Since cellular hypersensitivity (positive skin test and lymphocyte blastogenesis) develops at the same time as lesions reach maximum erythema, cellular hypersensitivity appears to be responsible for the intense inflammatory response associated with ringworm infections.