The antibacterial activities of 4 cephalosporin-type antibiotics, together with their corresponding derivatives having a methoxy group in the 7α-position, are reported. The compounds tested were cephalothin and 7α-methoxycephalothin, cefoperazone and 7α-methoxycefoperazone, cefoxitin and demethoxycefoxitin, cefamandole nafate and 7α-methoxycefamandole. Min. inhibitory concentrations were determined by the broth dilution method. The methoxylation of cephalothin results in an antibiotic (7α-methoxycephalothin) which, while maintaining its activity against gram-pos. bacteria, shows marked enhanced activity against gram-neg. bacteria, especially Alcaligenes faecalis, Escherichia coli, Proteus, and Shigella. The methoxylation of demethoxycefoxitin, an antibiotic with poor activity, gives rise to a well-known antibiotic (cefoxitin) with marked activity against both gram-neg. and -pos. bacteria. However, methoxylation in position 7α of cefamandole nafate and cefoperazone leads to less active compounds In general, methoxylation in position 7α is useful in those compounds which do not show resistance against the action of β. lactamases. In contrast, 7α-methoxylation of compounds which have side-chains giving partial or total stability against the action of β-lactamases may lead to compounds with less antibacterial activity.