Piperacillin phthalidyl ester was synthesized by reacting piperacillin with triethylamine and bromophthalide in acetone and its chemical structure was determined by UV, IR, and PMR. The partition coefficient of the ester was increased and the ester was more lipophilic and less water soluble than piperacillin. The ester did not show the antimicrobial activity against Bacillus subtilis ATCC 6633 in vitro, but when hydrolyzed, the parent drug of ester, piperacillin, revealed antimicrobial activity in vivo. After a single oral dose of both piperacillin and the ester to rabbits, the serum piperacillin concentration was measured by bioassay. The ester exhibited improved pharmatokinetic characteristics: $T_{max};of;2hr,;C_{max};of;4.26{mu}g{cdot}ml^{-1},K_{el};of;0.057hr^{-1},;and;total;AUC;of;85.42{mu}g{cdot}hr{cdot}ml^{-1}$. Piperacillin on the other hand, did not exhibit any gastro-intestinal absorption.