HR 756 is a new beta-lactamase resistant cephalosporin. The activity of HR 756 was compared in vitro to that of cefamandole, cefoxitine, cefuroxime and cephalothin. A concentration of 1 microgram/ml inhibited practically all isolates of E. coli, Salmonella, Shigella, Proteus indole + and indole --, Klebsiella, etc. A lower activity in vitro has been found regarding to Enterobacter, Serratia, Citrobacter and Pasteurella. The cefotaxime has some anti-Pseudomonas activity at concentrations of 7.5-15 micrograms/ml. For most Gram-negative organisms, HR 756 was most active. The cephalothin and the cefoxitine have resulted more active regard to Staph. aureus and Bacteroides fragilis, respectively.