Cephamycins, which have a methoxy group at the 7 alpha position of their cephalosporin nuclei, were highly stable against hydrolysis by a beta-lactamase purified from a clinical isolate of Morganella morganii, whereas their 7 alpha-H analogs were rapidly hydrolyzed by the enzyme. The high stability of the cephamycins was not due to their low affinity for the enzyme, since the cephamycins strongly inhibited the enzyme in a competitive manner. In cases in which the 7 alpha-methoxy group was substituted by OC2H5, SCH3, CN, or CH3 groups, the substituted compounds still showed high stability against enzymatic hydrolysis but significantly reduced both their affinities for the enzyme and their antibacterial activities. These 7 alpha-substituted cephalosporins, except for cephamycins, also had greatly reduced affinities for target proteins, i.e., penicillin-binding proteins, of beta-lactam antibiotics. Therefore, the 7 alpha position of cephalosporins was considered to play an important role in both beta-lactamase stability and antibacterial activity.