The in vitro activity of HR 756, a new cephalosporin, has been determined against recent clinical isolates and compared with that of other β-lactam antibiotics. The geometric means of the minimum inhibitory concentrations (MIC) for different isolates of Escherichia coli (100 isolates), Klebsiella pneumoniae (84), Pseudomonas aeruginosa (121), Proteus mirabilis (52), indole-positive Proteus species (9), Salmonella species (19), Staphylococcus aureus penicillinsensitive (29) and penicillin-resistant (39) were: 0.095, 0.124, 11.1, 0.095, 0.0107, 0.078, 1 and 0.95 mcg/ml, respectively. Its activity was affected by rise in inoculum against S. aureus and P. aeruginosa but not against K. pneumoniae and E. coli. Bactericidal activity was determined by membrane filtration method. HR 756 was found to be bactericidal to E. coli, K. pneumoniae, P. aeruginosa and Proteus species. Although the MICs of the tested S. aureus strains were 1 mcg/ml, 5 mcg/ml of HR 756 failed to kill 99% of the inoculum within 24 hours.