EXTRACTS of leaf tissue of broad bean (Vicia faba, L.) contain an enzyme which shows little tyrosinase (E.C. 188.8.131.52)1 activity unless treated with a denaturing agent2-5 when the activity toward both orthodiphenols and mono-phenols normally increases 50-fold. Conditions under which the denaturing agents are used for activation (Table 1) are normally considered to be insufficient to break covalent bonds associated with the protein molecules but are known to alter the tertiary structure and to cause some proteins to dissociate into sub-units6. Like other tyrosinases7 the isolated latent enzyme contains copper and exists in several forms separable by column chromatography or by electrophoresis on starch gel. Inhibition investigations have shown that the latent enzyme is less sensitive toward copper complexing agents, such as cyanide and diethyldithiocarbamate, than the active form, indicating that activation is probably accompanied by an unmasking of the prosthetic group.