An improved process for preparing the compound 7.beta.-(2-thienylacetamido)-7-methoxy-3-carbamoyloxymethyl-3-cephem-4-carboxylic acid or its esters, from the compound 7.beta.-(D-5-amino-5-carboxyvaleramido)-3-carbamoyloxymethyl-7-methoxy-3-cephem-4-carboxylic acid, by conducting the transacylation of the latter compound in the presence of commercially available alumino-silicate zeolites, also known as "molecular sieves." The process can be employed to prepare a cephalosporin with a desired 7-acylamido group from cephalosporins having a broad range of different 7-acylamido groups without having to isolate and purify a 7-amino intermediate. The final products have utility as broad spectrum antibiotics.