The localization and activity of polyamine oxidase (PAO; EC 18.104.22.168), was investigated in leaves and protoplasts of oat seedlings. Activity of the enzyme is highest with spermine as substrate; spermidine is also oxidized, but putrescine and cadaverine are unaffected by the enzyme. Protoplasts isolated following digestion of leaves with cellulase in hypertonic osmoticum showed no PAO activity, and about 80% of the total leaf PAO activity could be accounted for in the cell wall debris. Histochemical localization experiments showed intense PAO activity in guard cells and in vascular elements whose walls are not digested by cellulase. When protoplasts were cultured in a medium suitable for regeneration of cell wall, PAO activity could be detected as the cellulose wall developed. Thus, PAO appears to be localized in cell walls.