Ringworm of the scalp of children caused by M. audouini clears up spontaneously during adolescence, obviously because of the pubertal development of sebaceous glands. If hair fat of adults is added to Sabouraud''s medium in a conc. of 1:200, growth of M. audouini is prevented. Adult hair fat was fractionated and the activity of the fractions was tested quantitatively on M. audouini cultures. The final fraction inhibited the growth of M. audouini in a conc. of 1:200,000. Preliminary chemical analyses indicated that the activity is due to the presence of aliphatic satd. fatty acids between C7 and C11.