A new antibiotic compound, gephyronic acid was isolated from the culture broth of the myxobacterium, Archangium gephyra strain Ar 3895. Up to Smg/liter was produced during the logarithmic and stationary growth phase. The compound is an aliphatic acid, which tends to form a hemiacetal. Both forms inhibited growth of yeasts and molds (MIC 1-25μg/ml) and had a cytostatic effect on mammalian cell cultures (IC₅₀ 10-60ng/ml). Gephyronic acid is a specific inhibitor of eukaryotic protein sythesis showing an IC₅₀ of 1-2 × 10^-7mol/liter in an in vitro translation assay.