Abstract: A 6.beta.-substituted penicillanic acid of the formula XII: ##STR1## or a pharmaceutically acceptable salt or ester thereof, whereinA and B are each hydrogen wherein the carbon atoms to which A and B are attached are linked by a single bond, or A and B together form a bond, wherein the carbon atoms to which A and B are attached are linked by a double bond,R1 is selected from the group consisting of a) hydrogen, b) a pharmaceutically acceptable salt and c) a pharmaceutically acceptable group selected from the group consisting of C.sub.1-10 alkyl, C.sub.2-10 alkenyl, C.sub.2-10 alkynyl, C.sub.3-10 cycloalkyl, C.sub.