3-Carbamoyloxymethyl and 3-N-methylcarbamoyloxymethyl cephalosporin antibiotics in which the 7.beta.-acylamido group has the structure ##STR1## where R is thienyl, furyl or phenyl; R.sup.a is C.sub.1-4 alkyl, C.sub.2-4 alkenyl, C.sub.3-7 cycloalkyl or phenyl, and R.sup.b is hydrogen, carboxy, C.sub.2 -C.sub.5 carbalkoxy or any of the groups designated for R.sup.a, or R.sup.a and R.sup.b together with the carbon atom to which they are attached form a C.sub.3-7 cycloalkylidene or cycloalkenylidene group; exhibit broad spectrum antibiotic activity characterized by particularly high activity against gram negative microorganisms, including those which produce .beta.-lactamases. The compounds, which are syn isomers or exist as mixtures of syn and anti isomers containing at least 90% of the syn isomer, have particularly high in vitro activity against strains of Escherichia coli, Haemophilus influenzae and Proteus organisms; and also shown unusually high activity against Pseudomonas organisms.