biological evaluation of a series of 7-[2-(2-aminothiazol-4-yl)-2-(Z)-[(cyclopentyloxy)imino]acetamido] opticallyactive 2-oxaisocephems, substituted at the 3-position with [(N-alkylpyridinium-4'-yl)thio]methyl groups, are described. The resulting family of parenteral compounds displays a broad spectrum of in vitro antibacterialactivity. These compounds exhibit increased activity against Gram-positive organisms