6.beta.-[2-(2'-methylphenoxycarbonyl)-2-thien-3'-ylacetamido] penicillanic acid of formula (II): ##STR1## and pharmaceutically acceptable salts and in vivo hydrolyzable esters thereof, are active against Gram-positive and Gram-negative bacteria which makes them useful as therapeutic and prophylactic agents against bacterial infections in animals, including man and poultry. A process for the preparation and pharamceutical compositions comprising the penicillin, are also described.