The kinetics of the activation of catechol by tyrosinase prepared from the potato and the mushroom, and of its inhibition by sodium m-hydroxybenzoate, have been studied. The enzyme-substrate dissociation constants differed markedly between the two enzyme sources (K. potato = 5.OmM, Kg mushroom = O.28mM), as did also the enzyme-inhibitor dissociation constants (K; potato = 2.5mM, Ki mushroom = O.BmM). For both enzyme preparations sodium m-hydroxybenzoate met the requirements of a competitive inhibitor.