BB-K8, an aminoglycoside antibiotic which is a derivative of kanamycin, was tested in vitro against 466 clinical bacterial isolates. Over 90% of gram-negative bacilli, except Proteus spp., were inhibited by 3.12 mug of BB-K8 per ml. This antibiotic was consistently more active than kanamycin but less active than tobramycin or gentamicin. Unlike kanamycin, BB-K8 was active against Pseudomonas aeruginosa. Eleven of 19 isolates resistant to either gentamicin or tobramycin, or both, were susceptible to BB-K8.