Novel antibacterial antibiotics, amythiamicins A, B, C and D, have been isolated from the fermentation broth of Amycolatopsis sp. MI481-42F4. In this paper, the taxonomy of the producing strain, fermentation, isolation, physico-chemical properties and biological activities of amythiamicins are reported. Amythiamicins inhibit the growth of Gram-positive bacteria including multi-drug resistant strains.