Cephalosporins having an .alpha.-acyloxyacetic acid side chain

A compound of the formula ##STR20## wherein R.sub.1 denotes phenyl, thienyl, furyl, 1,4-cyclohexadienyl, or phenyl substituted by methyl, lower alkoxy, hydroxy, halogen or nitro, R.sub.2 represents a free carboxyl group or a pivaloyloxymethoxycarbonyl group, R.sub.3 represents a heterocyclylthiomethyl group wherein the heterocyclyl radical is 1-oxidized pyridyl, pyrimidyl, pyridazinyl, pyrazinyl, imidazolyl, imidazolidyl, purinyl, triazolyl, tetrazolyl or such radicals substituted by lower alkyl, lower alkoxy, hydroxy or halogen, and B represents the formula (B.sub.1) ##STR21## wherein X.sub.1 and X.sub.2 independently of one another denote oxygen, sulphur or the imino group .dbd.NH, R.sub.4 and R.sub.5 independently of one another denote hydrogen or methyl, and R.sub.6 denotes hydrogen, halogen, methyl or phenyl, or represents the formula (B.sub.2) ##STR22## wherein R.sub.6 denotes hydrogen, halogen, methyl or phenyl, and R.sub.7 and R.sub.8 independently of one another denote a free hydroxyl or mercapto group, lower alkoxy, lower alkyl mercapto, lower alkanoyloxy or lower alkanoyl mercapto, halogen or a mono- or di-lower alkylated or lower alkanoylated amino group, and one of the radicals R.sub.7 or R.sub.8 can also be hydrogen, or represents the formula (B.sub.5) ##STR23## wherein R.sub.20 denotes a free hydroxyl or mercapto group, lower alkoxy, lower alkyl mercapto, lower alkanoyloxy or lower alkanoyl mercapto, and R.sub.21 denotes hydrogen, hydroxyl, halogen or methyl, or represents the formula (B.sub.6) ##STR24## wherein R.sub.22 denotes a free hydroxyl or mercapto group, lower alkoxy, lower alkyl mercapto, lower alkanoyloxy, or lower alkanoyl mercapto, and R.sub.23 denotes hydrogen, hydroxyl, halogen or methyl, or represents, where appropriate, a formula tautomeric thereto, or a pharmaceutically acceptable salt thereof.