The synthesis and in vitro and in vivo activities of a series of 7-sulfonylacetamido-3-cephem-4-carboxylic acids with acetoxymethyl or heterocyclic thiomethyl substituents at the 3 position are described. Lengthening the alkyl chain attached to the sulfonyl group increased gram-positive activity but the effect on gram-negative activity was variable. Other structural changes on the 7-acyl side chain resulted in only minor changes in vitro activity. the protective effectiveness in infected mice generally paralleled the in vitro activity, except that the butylsulfonyl derivatives were less protective than predicted by in vitro activity. replacement of the 3-acetoxymethyl by a 3-heterocyclic thiomethyl group resulted in an overall improvement of activity both in vitro and in vivo.