Antimycin A3 is a homologue of the antimycin complex, first isolated in 1958 from Streptomyces blastimyceticus and co-produced with antimycin A4. Antimycin A3 is a more polar analogue of A1 and A2. Like all antimycins, antimycin A3 is a potent inhibitor of respiration by inhibiting the oxidation of ubiquinol to ubiquinone. Antimycin A3 exhibits broad biological activity as an antifungal, anthelmintic, insecticidal, antiviral and antitumor active. Lack of availability of the individual antimycin components has hindered greater understanding of the common and unique molecular targets of the components of the antimycin complex.