A partial characterization of the primary structure of D-amino-acid oxidase (D-Amino-acid:oxygen oxidoreductase (deaminating), EC 1.4.3.3.) from hog kidney has been achieved by a CNBr cleavage of the 14C-carboxymethylated protein. Four fragments have been isolated and purified and their alignment made possible by overlapping with methionine-containing peptides derived from tryptic digestion of the 14C-carboxymethylated protein. A partial sequencing of the CNBr fragments has been carried out by the automated Edman procedure and by manual sequence analysis. Chymotryptic peptides containing the 5 alkylated thiols of the monomer enzyme (Curti, B., Ronchi, S., branzoli, U., Ferri, G. and Williams, Jr., C. H. (1973) Biochim. Biophys. Acta 327, 266-273) have been isolated and their sequence determined. The present results do not show any significant homologies with the known sequences of other flavoproteins.