Abstract: Tricyclic compounds of formula (I) ##STR1## wherein X.sup.1 is a carboxyl or 5-tetrazolyl groupX.sup.2 is carbonyl or methyleneX.sup.3 is hydroxyl or a group --X.sup.4 (C.sub.n H.sub.2n)X.sup.5whereX.sup.4 is oxygen or sulphurX.sup.5 is hydrogen or a group --OX.sup.6whereX.sup.6 is hydrogen, alkanoyl of 1 to 4 carbon atoms or a group --(C.sub.m H.sub.2m)X.sup.7whereX.sup.7 is hydrogen or a group --OX.sup.8whereX.sup.8 is hydrogen or alkanoyl of 1 to 4 carbon atoms and m and n are each, independently, an integer from 1 to 4, together with salts thereof,provided that when X.sup.5 is a group --OX.sup.6 then n is always greater than 1 and X.sup.4 and X.sup.5 are attached to different carbon atomsand that when X.sup.7 is a group --OX.sup.8 then m is always greater than 1 and no single carbon atom in the radical --(C.sub.m H.sub.2m)-- is attached to two oxygen atoms.