Of 52 patients with proved Trichophyton tonsurans infection of the scalp, 16 had an inflammatory reaction to this infection (kerion), while the rest had only slight scaling and broken hairs (black dot ring-worm). Each patient was tested with trichophytin antigen to determine whether cell-mediated immunity (CMI) was responsible for the differences in these different clinical presentations. Fifteen of 16 with kerion had a 48-hour reaction greater than 10 mm, while none of the 36 with noninflammatory (black dot) tinea capitis had a similar response. This suggests that the patient's immune response (CMI) may be responsible for kerion infections of the scalp and that therapy could be directed initially toward suppressing this reaction.