Twenty-six keratinolytic fungi (16 dermatophytes and 10 soil species) and sixteen non-keratinolytic fungi were cultivated on sterile human hair in a mineral solution. With keratinolytic fungi, the loss in total dry weight (hair + mycelium) reached 7.6 up to 24.2% after 60 days of cultivation. Peptidic (Lowry-positive) substances accumulated in the medium and pH of the medium gradually increased, often over pH 8. Excess of sulfur contained in the substrate was excreted back into the medium predominantly in the form of inorganic sulfate. Moreover, sulfite was produced that cleaved disulfide bonds of the substrate giving rise to S-sulfocysteine; soluble products of keratin degradation in the medium contained 2.1 to 6.5% of that substance. This supports the presumption that sulfitolysis of disulfide bonds is a key reaction of keratinolysis in fungi. Statistically significant correlations were observed between substrate degradation and medium alkalinization, as well as the contents of peptidic substances, sulfate, and S-sulfocysteine. The correlation was highest with sulfate content and least significant with peptidic substances. Non-keratinolytic fungi mostly grew on the hair but did not cause a gravimetrically measurable loss of the substrate, did not alkalinize the medium, and did not accumulate the above mentioned substances in amounts comparable with those of keratinolytic species.