A series of novel 7-(D-.alpha.-acylaminoarylacetamido)-.DELTA..sup.3 -cephem derivatives have been prepared wherein the acyl moiety contains an epoxy group immediately adjacent to the carbonyl carbon atom. These compounds are useful as antibacterial agents for the treatment of diseases caused by Gram-negative and Gram-positive bacteria. Preferred members include 7-[D-.alpha.-(cis-2-carboxyoxiran-3-carboxamido)phenylacetamido]-3-(1-methyl-1,2,3,4-tetrazol-5-ylthiomethyl)-.DELTA..sup.3 -cephem-4-carboxylic acid and 7-[D.alpha.-(cis-2-carboxyoxiran-3-carboxamido)phenylacetamido]cephalosporanic acid. Alternate methods of preparation are provided for these compounds and the principal synthetic route is described in detail.