Abstract: Disclosed are novel ethylidene derivatives of tricyclic carbapenems of the formula I wherein the ring marked C and X have the meaning as in the description, in the form of pure diastereoisomers and in the form of pure geometric isomers. Novel ethylidene derivatives of tricyclic carbapenems of the formula I are used as inhibitors of the action of the enzyme &bgr;-lactamase and/or as antibiotics in human and veterinary medicine. Also disclosed are pharmaceutical formulations for the treatment of bacterial infections of human and animal organisms comprising as an active ingredient a therapeutically effective amount of an ethylidene derivative of tricyclic carbapenems of the formula I in the form of an acid, a pharmaceutically acceptable salt or ester thereof and optionally in a combination with a &bgr;-lactam antibiotic, and usual pharmaceutically acceptable carriers and auxiliary substances.