In the course of screening for aminoglycoside antibiotics, new members of the fortimicin and sporaricin group, sannamycins A and B, have been isolated from a culture broth of Streptomyces sp. KC-7038 which was classified as Streptomyces sannanensis sp. nov. The strain KC-7038 was cultured in a medium composed of 4% corn starch, 0.2% soy bean meal, 0.5% corn steep liquor, 0.2% yeast extract and inorganic salts (0.3% NaCl, 0.1% CaCO3, 0.05% MgSO4·7H2O) at 27°C for 4 days. Fermentation broth was processed with cation-exchange resin Amberlite IRC-50, CM-Sephadex C-25, and cellulose column chromatography to obtain pure sannamycins A (98 mg) and B (45 mg). The molecular formulas of sannamycins A and B were C17H35N5O5 and C15H32N4O4, respectively, as determined by mass spectrometry and elementary analysis. Their physico-chemical properties including specific rotation, IR, 1H NMR, and mass spectra were characterized. Sannamycin A was highly active against Gram-positive and Gram-negative organisms including various aminoglycoside-resistant strains, with slightly weaker activity than sporaricin A, while sannamycin B showed low activity. Intravenous acute LD50 of sannamycins A and B in mice were 100~200 mg/kg and >400 mg/kg, respectively. The structures of sannamycins A and B were established, and a detailed account of structure elucidation will be presented in a separate paper.