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A latent phenoloxidase (PPO) was purified in the presence of protease inhibitors from carrot cells to electrophoretic homogeneity. The inactive enzyme had a M(r) of ca 59 000 under denaturating conditions as judged by SDS-PAGE. When stored in the absence of protease inhibitors, PPO in a crude extract was converted to forms with a lower M(r). Phenol oxidase activity of the purified PPO was induced by the presence in the assay medium of 12.5 mM Tris, and 6 mM CaCl2 increased the activity further.