The structure of tetrafibricin, a novel and potent fibrinogen receptor antagonist isolated from the culture broth of Streptomyces neyagawaensis NR0577, was determined. Tetrafibricin has a unique structure containing primary amine, conjugated tetraenoic acid, and polyhydroxy functionalities that is biosynthetically related to the polyene macrolide antibiotics. As described in the preceding paper, tetrafibricin was a new metabolite produced by Streptomyces neyagawaensis NR0577, which potently inhibited fibrinogen binding to its receptors.