From the cell mass and culture supernatant of Myxococcus xanthus strain Mx X12 an antibiotic activity against yeasts, molds and some Gram-positive bacteria could be extracted. It consisted of 4 biologically active compounds which were named myxalamid A, B, C and D. The main component, myxalamid B, was shown to block in beef heart submitochondrial particles the respiratory chain at the site of complex I, i.e. NADH: ubiquinone oxidoreductase. The myxalamids are new antibiotics.