A new, efficient process is provided for the production of 7.beta.-[(2D-2-amino-2-carboxy)-ethylthio-acetamido]-7.alpha.-methoxy-3-[(1-methyl-1H-tetrazole-5-yl) thiomethyl]-3-cephem-4-carboxylic acid useful as new antibacterial agent. This process is economic in using as the starting material the inexpensive, corresponding cephem compound containing no 7.alpha.-methoxy group on the cephem nucleus and comprises 7.alpha.-methoxylation of a protected derivative of the starting cephem compound with t-butyl hypochlorite and lithium methoxide, followed by inactivation of the excessive methoxylation reagents with a trialkyl phosphite and acetic acid to prevent undesired side-reactions such as oxidation of the alkylthioacetyl group of the product, and further by conventional removal of the protecting groups.