The following results were obtained from the bacteriological evaluations of netilmicin (NTL), a newly developed antibiotic agent, with gentamicin (GM), dibekacin (DKB) and amikacin (AMK) as the controls. (1) NTL demonstrated broad antibacterial spectra against both Gram-positive and Gram-negative bacteria, but its antibacterial potency against streptococci was not very strong among other Gram-positive bacteria. (2) In terms of distribution of sensitivity of clinically isolated bacterial strains, NTL proved to have antibacterial potency comparable to that of GM and higher potency than that of DKB of AMK against E. coli K. pneumonia, Enterobacter sp., or H. influenzae. However, its efficacy was inferior to GM against Proteus sp., S. marcescens and P. aeruginosa. (3) In conjunction with the influences of pH of culture media or of addition of horse sera upon the antibacterial efficacy, NTL showed an inclination similar to that of GM, DKB and AKM. Its antibacterial efficacy was fortified on the alkaline side or by addition of sera. In connection with the influences of the amounts of inoculated bacteria upon antibacterial efficacy, there were hardly any appreciable influences on it by any of the tested bacterial strains. (4) The interactions of NTL with carbenicillin were evaluated with the chequerboard titration method to find remarkable cooperative actions in any of E. coli, K. pneumoniae, S. marcescens, A. calcoaceticus and P. aeruginosa. (5) The results of evaluation on the patterns of its antibacterial effects revealed that it acted bactericidal in any tested bacterial strains. (6) As to the therapeutic effects against experimental infections in mice, it was found out that NTL = GM greater than DKB and AMK against E. coli, GM greater than NTL = DKB and AMK against K. pneumoniae and GM and DKB greater than NTL greater than or equal to AMK against A. calcoaceticus and P. aeruginosa in the decreasing order of efficacy.