A culture of Micromonospora species MK-70 was found to produce two new antibiotics, fortimicins A and B. Antibacterial and paper chromatographic data on an eluate from IRC-50 treatment of fermentation beers indicated that fortimicins A and B are new antibiotics with broad-spectrum, basic and water-soluble properties. Fortimicin A exhibited potent, unique, broad-spectrum antibacterial activity against Gram-positive and negative bacteria both in vitro and in vivo, while fortimicin B was only weakly active.