Novel penicillin derivatives of formula [I] ##STR15## wherein A is hydrogen or hydroxyl, and B represents the coumarin nucleus of formula [III-a] ##STR16## in which B.sub.1, B.sub.2, B.sub.3, B.sub.4 and B.sub.5 are the same or different, and are selected from the group consisting of hydrogen, hydroxyl, thiol, acetyl, acetyloxy, amino, carboxyl, chloro, bromo, alkoxy of 1 to 4 carbon atoms, alkylmercapto of 1 to 4 carbon atoms, alkylamino of 1 to 8 carbon atoms, alkoxycarbonyl of 2 to 5 carbon atoms and nontoxic carboxylic salt, or nontoxic salts thereof.