Recently various new aminoglycoside derivatives were obtained by a technique called "mutational biosynthesis" or "mutasynthesis".1~3) However, most of them were derivatives which could be produced by the action of deoxystreptamine-negative mutants of aminoglycoside producing strains (idiotrophs) upon addition of some synthetic analogs of deoxystreptamine to the medium. That is, they were aminoglycosides in which the deoxystreptamine moiety was replaced with those synthetic analogs.4~7, 1, 8~13) By noting the fact that most of gentamicins are 3',4'-dideoxy compounds, we attempted to utilize gentamicin producing organisms or their mutants for the conversion of kanamycins to 3',4'-dideoxykanamycins. To our surprise, we obtained a new series of compounds which are characterized by 4"-C-methylation as well as 3',4'-dideoxygenation in the kanamycin molecule. The new series of compounds, combimicins (named after their hybridized structure of kanamycins and gentamicins), were obtained by growing a new gentamicin producer, Micromonospora sp. K-6993 which was isolated by us from a soil sample, or its gentamicin non-producing mutant Y-41 (registered as ATCC 31348 and 31349 respectively) in the presence of kanamycins. From kanamycin B were obtained combimicin B1 (I) and combimicin B2 (II), and from kanamycin A was derived combimicin A2 (III).