Prior to initiation of cefoperazone therapy, appropriate specimens should be obtained for identification of the causative organism and in vitro susceptibility tests. Cefoperazone may be started pending results of susceptibility tests but should be discontinued if the causative organism is found to be resistant to the drug. In serious infections when the causative organism is unknown, concomitant therapy with an aminoglycoside may be indicated pending results of in vitro susceptibility tests. However, in vitro studies indicate that cefoperazone is less active on a weight basis than cefotaxime, ceftizoxime, and ceftriaxone against susceptible Enterobacteriaceae, and other cephalosporins (e.g., cefepime, cefotaxime, ceftazidime, ceftriaxone, ceftizoxime) generally are preferred when a parenteral cephalosporin is indicated for the treatment of infections known or suspected to be caused by Enterobacteriaceae.