Antimycin A4 is the most polar of the major components of the antimycin complex, first isolated in 1958 from Streptomyces blastimyceticus and co-produced with antimycin A3. Like all antimycins, antimycin A4 is a potent inhibitor of respiration by inhibiting the oxidation of ubiquinol to ubiquinone. Antimycin A4 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.