Diaminobenzidine (DAB) histochemistry was used to determine enzymatic or non enzymatic peroxidase activity in the rat limb bud on days 12 and 13 of gestation. The optimal technical conditions were determined. Light microscopy revealed the presence of positive intracellular granules in the apical ectodermal ridge (A.E.R.) and in the condensed mesoderm. Their distribution in the A.E.R. shows a caudo-cephalic gradient. Electron microscopy, red oil red, and supravital Nile blue staining suggest the lipidic nature of these granules. The hypothesis of an endogenous pseudo- or peroxidase activity are discussed.