Diaminobenzidine (DAB) was used in the ultrastructural localization of oxidized DAB reactive sites in chloroplasts of Nicotiana tabacum flower pedicel abscission zone tissue. The chloroplast thylakoids and membrane-bound body, in light-incubated tissue, at pH 9.0, show evidence of DAB reactive sites. Tissue preincubated in 0.02 M potassium cyanide, prior to incubation in DAB medium containing the inhibitor, show a marked decrease in DAB staining in both the thylakoids and membrane-bound body of the chloroplast. However, tissue incubated in DAB medium containing 0.02 M 3-amino-1,2,4-triazole, a catalase inhibitor, reveals no inhibition of DAB staining in either the chloroplast thylakoids or membrane-bound body. Control tissue incubated in DAB medium without H2O2 or in complete medium without DAB, demonstrate an absence of DAB oxidation product in both the chloroplast thylakoids and membrane-bound body. However, tissue incubated in DAB medium at pH 6.0 have DAB staining only in the mitochondrial membranes, due to cytochrome c, with no DAB staining evident in the chloroplast thylakoids and membrane-bound body. It is suggested that the chloroplast thylakoid staining is due to the photo-oxidation of DAB, whereas the DAB reaction product in the membrane-bound body indicates peroxidase localization. These findings are discussed in terms of the possible involvement of the chloroplast membrane-bound body in thylakoid formation, photosystems, and protein storage.