Abstract: This invention relates to penicillanic acid derivatives of the formula I ##STR1## in which X stands for chlorine, bromine or iodine, to pharmaceutically acceptable, non-toxic salts of the compounds of formula I, to pharmaceutically acceptable, easily hydrolyzable esters thereof, including salts of such esters, to pharmaceutical compositions containing the compounds of the invention and dosage units thereof, to methods for the preparation of the compounds of the invention, and to methods of using the said new compounds in the human and veterinary therapy.The 6.beta.-halopenicillanic acids of formula I are potent inhibitors of .beta.-lactamases from a variety of gram-positive and gram-negative bacteria, making the 6.beta.-halopenicillanic acids as well as their salts and easily hydrolyzable esters valuable in human and veterinary medicine.