Peroxidase activity has been localized in the cell walls and cytoplasm of wound vessel elements of Coleus which had been fixed in glutaraldehyde, incubated in diaminobenzidine (DAB) and H2O2, and postfixed in OSO4. Electron microscopic investigations revealed prominent staining in the reticulate secondary wall and in the primary wall where the secondary thickenings attach. The stain in the secondary wall is finely textured and heavier towards its periphery than towards its core. The staining of the primary wall, however, is coarsely granular. In the cytoplasm of differentiating vessel elements electron-opaque deposits are observed in the plasmalemma, especially where it overlies the secondary thickening, and in the dictyosomes and their associated vesicles. Staining also occurs on the internal membranes of developing chloroplasts where it is most likely the result of photooxidation of DAB.Staining, except in chloroplasts, appears to be due specifically to peroxidase, since either removal of H2O2 or preincubation with KCN markedly reduces staining, whereas preincubation with aminotriazole, an inhibitor of catalase, does not. The similarity of localization of peroxidase and lignin in the walls of Coleus wound vessel elements supports the postulate that the enzyme participates in lignification.