Chemical and physical evidence are combined in establishing the constitution of the antimycins (XVII), the streptomyces antibiotic group noted for its effect in inhibiting electron transport mechanisms in aerobic systems. Specific structures are proposed for antimycin A₁ and antimycin A₃, two components of the antibiotic complex. A generalized formula for the antimycins is presented based upon findings secured in these laboratories which represent the totality of the pertinent chemical information available up to the present time.