The stability of piperacillin sodium in the presence of tazobactam sodium in 5% dextrose and normal saline i.v. admixtures has been determined using a modified stability-indicating high performance liquid chromatography assay method reported in the literature. The solutions were stored at room and refrigerator temperatures in plastic bags. They remained clear throughout the study although the pH values decreased slightly during storage. The solutions were stable for 2 days at 25 degrees C and for 28 days at 5 degrees C. Tazobactam sodium appears to have a slight adverse effect on the stability of piperacillin sodium.