The localization of sex-specific cytochrome P-450, P-450 (M-1), was investigated immunohistochemically in the liver of untreated- and Aztreonam-treated male rats. P-450 (M-1) was uniformly distributed in the liver lobule of untreated rats. By treatment with Aztreonam, P-450 (M-1) was decreased more in the periportal region rather than the pericentral region of the lobule. In addition, oxidation of testosterone and 1 alpha-hydroxycholecalciferol catalyzed by P-450 (M-1) was also decreased by Aztreonam administration.