The antibacterial activities of K-4619 (3-de-O-methylsporaricin A sulfate) were compared with those of sporaricin A, amikacin, dibekacin, and gentamicin. K-4619 exhibited a high order of activity against gram-positive and gram-negative bacteria, including Pseudomonas aeruginosa. Its activity against Providencia species and Serratia marcescens was the highest of all drugs tested. K-4619 was highly effective against bacteria that produce various aminoglycoside-inactivating enzymes, except for 3-acetyltransferase I. The bactericidal activity of K-4619 was somewhat greater than that of amikacin. The activity of K-4619 against gram-negative bacteria increased at alkaline pH and was hardly affected by inoculum size, addition of horse serum, and composition of the medium. The in vivo protective effect of K-4619 against infections with Klebsiella pneumoniae, S. marcescens, and P. aeruginosa in mice was greater than that of sporaricin A. K-4619 was also active in mice infected with gentamicin- or amikacin-resistant strains bearing some of the aminoglycoside-inactivating enzymes.