Abstract: A compound of the formula ##STR1## and pharmaceutically acceptable salts thereof, in which A is --N.dbd.N--NH--, --X--C(R.sub.2).dbd.N--, --N.dbd.C(R.sub.2)--X--, --X--C(R.sub.2)--CH(R.sub.2).dbd.N-- or --N.dbd.C(R.sub.2)--CH(R.sub.2)--X--, wherein X is sulphur, oxygen or nitrogen and R.sub.2 is provided that where two R.sub.2 -groups are present in the A group one of the R.sub.2 groups is hydrogen, amino, substituted amino, C.sub.1-6 alkyl, substituted acylamino or one of the following groups: ##STR2## wherein R.sub.a is hydrogen, amino, substituted amino, C.sub.1-6 alkoxy, heterocyclic five membered ring or --NHCOR', wherein R' is C.sub.1-6 alkyl, aryl or heterocyclic five membered ring, R.sub.b is hydrogen, C.sub.1-6 alkoxy, C.sub.1-6 alkylthio or hydroxy, R.sub.c is hydrogen, C.sub.1-6 alkyl or oxygen and R.sub.d is C.sub.1-6 alkyl or oxygen, R.sub.e is hydrogen, hydroxy or C.sub.1-6 alkoxy, R.sub.f is C.sub.1-6 alkyl, provided that where two R.sub.2 -groups are present in the A group, one of the R.sub.