Replacement of a sulphur atom by an oxygen at the 1-position of the cephem nucleus generally resulted in fourfold to sixteenfold increase of antibacterial activity in each pair of the structural congeners. However, the increased antibacterial activity caused by the replacement was accompanied by instability to <latex>$\beta$</latex>-lactamase to some extent, which was due presumably to the increased chemical reactivity of the <latex>$\beta$</latex>-lactam ring system. The aim of the research effort is to confer <latex>$\beta$</latex>-lactamase stability and expand the Gram-negative spectrum. Two types of substituents have been demonstrated to protect 1-oxacephem from enzymic hydrolysis and their protecting effects were specifically related to the types of <latex>$\beta$</latex>-lactamases derived from Gram-negative bacteria: the 7<latex>$\beta$</latex>-malonylamino function is specific to cephalosporinase and the 7<latex>$\alpha$</latex>-methoxy group to penicillinase. The complementary effect of these substituents was clearly demonstrated. This line of studies led us to prepare the clinical candidate 6059-S, which possessed widely expanded antibacterial spectra against Gram-negative bacteria including indole-positive Proteus, Enterobacter, Sarratia marcescens, Pseudomonas aeruginosa and Bacteroides fragilis.