The heterogeneity of Vicia faba preparations is shown to be unrelated to the effect of denaturing agents. Conditions for maximal promotion of the latent tyrosinase activity have been studied and anionic detergents are shown to be better activating agents than urea and acid treatments. It is suggested that the activation process involves a limited conformational change such as a swelling of the protein rather than a dissociation or aggregation. In high concentrations of urea the cresolase activity is less easily inhibited than catecholase. The urea activated enzyme had a somewhat lower affinity for p-cresol than the manoxal activated enzyme. Guanidine salts are also activating agents but frequently inhibition of the activated form is encountered. Guanidine hydrochloride produces a stronger inhibition than sodium fluoride.