The important structural feature of the new cephalosporin derivative, cefotaxime, is the conjunction in the acetyl side - chain of the aminothiazolyl ring and the - syn - methoxyimino group. The aminothiazoloacetyl side - chain improves antibacterial properties against Gram negative bacilli, while the adjunction of the methoxyimino substituent is responsible for the further unexpectedly large increase in activity.