Cephalothin, a first generation cephalosporin for parenteral use was the first cephalosporin to become available for patients in the US in 1964. It was chosen for clinical trials from series of 7-ACA derivatives prepared at Eli Lilly. The second cephalosporin for parenteral use became available little later and was marketed in the US under the name Cephaloridine. The clinical successes of these two cephalosporins urged researchers to improve the pharmacological properties and develop more agents. Today we are left with thousands of semisynthesized analogues of natural cephalosporin compounds based on the knowledge gained by intensive research on the chemistry of those two starting materials.