Hydrogen peroxide generation by polyamine oxidation in the apoplast of some members of Leguminosae and Gramineae was investigated using biochemical, histochemical and immunohistochemical methods. Transversal sections obtained from the basal internode of light-grown epicotyls of Cicer arietinum L. and from mesocotyls of Zea mais L. were tested histochemically for peroxidase activity. The sections, incubated in syringaldazine or 4-chloro-l-naphthol, plus H202, showed a strong staining of the apoplast of sclerenchymatous fibers, endodermis (maize mesocotyl), xylem and vascular parenchyma. A positive reaction was also observed in the cell walls of the epidermis. All these structures, except in the epidermis, were positive with the phloroglucinol/HCI test for lignin. Control sections, incubated with catalase plus syringaldazine or 4-chloro-l-naphthol showed no staining. Addition of putrescine (chick-pea) or spermine (maize) to these sections gave the same histochemical pattern as observed after addition of hydrogen peroxide. These results indicate the occurrence of both peroxidases and amine oxidases in the apoplast of the above-mentioned tissues. Diamine oxidase occurrence in the apoplast of different tissues of chick-pea epicotyls was further confirmed by immunohistochemistry. These results suggest an involvement of polyamine oxidation in the synthesis of hydrogen peroxide possibly utilized by wall isoperoxidases for lignification and wall stiffening.