A new antibiotic, naphthyridinomycin, was isolated in crystalline form from the culture filtrate of Streptomyces lusitanus AY B-1026. The antibiotic is active against a large number of both gram-positive and gram-negative bacteria, and inactive against Candida albicans, Trichophyton granulosum and Microsporum gypseum. The antibiotic is toxic in mice.