Abstract: Cephalosporin antibiotics of formula ##STR1## (where R is thienyl or furyl; R.sup.a and R.sup.b are each selected from hydrogen, C.sub.1-4 alkyl, C.sub.2-4 alkenyl, C.sub.3-7 cycloalkyl, phenyl, naphthyl, thienyl, furyl, carboxy, C.sub.2-5 alkoxycarbonyl and cyano, 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; m and n are each 0 or 1 such that the sum of m and n is 0 or 1; and P is hydrogen or halogen) and their non-toxic derivatives 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; compounds wherein at least one of R.sup.a and R.sup.