The in vitro activity of HR 756, 7-[2-(2-amino-4-thiazolyl)-2-(Z)-(methoximino)acetamido] cephalosporanic acid, was investigated against 659 isolates. HR 756 inhibited Neisseria and Haemophilus species at concentrations similar to those needed with ampicillin. It inhibited β-lactamase-producing N. gonorrhoeae and H. influenzae . HR 756 was the most active compound tested against members of the Enterobacteriaceae , inhibiting most isolates of Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Salmonella, Enterobacter , and Shigella at concentrations of less than 0.1 μg/ml. It was twice as active as carbenicillin against Pseudomonas aeruginosa and inhibited Bacteroides fragilis as well as cefoxitin. HR 756 killed E. coli, Staphylococcus aureus , and P. aeruginosa at rates similar to other β-lactam antibiotics.