Treatment of fortimicin B with lithium in ethylamine gave 3-O-demethylfortimicin B. The latter was converted by methodology developed with fortimicin B to 3-O-demethylfortimicin A, 4-N-sarcosyl-3-O-demethylfortimicin B, 4-N-beta-alanyl-3-O-demethylfortimicin B, and 4-N-(beta-aminoethyl)-3-O-demethylfortimicin B. 3-O-demethylfortimicin A and the 4-N-acyl-3-O-demethylfortimicins B had appreciably higher antibacterial activities than the corresponding parent fortimicins. Most significant was the increased activity of 3-O-demethylfortimicin A relative to fortimicin A against a variety of strains of Pseudomonas aeruginosa.