The effects of Myceliophthora thermophila laccase are studied alone and in combina tion with the mediators violuric acid (VA) or l-hydroxybenzotriazole (HBT) on wool fibers, cystine, and tyrosine. Without a mediator, laccase is unable to oxidize wool or the amino acids. In the presence of either VA or HBT, oxygen consumption is observed, indicating oxidation of the secondary substrates of the laccase, i.e., wool, cystine, or tyrosine. The oxidation levels are low. Laccase/HBT reacts more efficiently with wool and tyrosine and laccase/VA with cystine. With the chosen enzyme dosage and mediator concentration, about 2-4% of cystine and about 1 % of tyrosine are oxidized. The slight oxidation of wool fibers is not sufficient to affect the surface chemistry or alkaline solubility of the fibers, although staining of the fibers by the oxidized mediators is significant.