Kinetic studies of cytochrome reduction show that only two beta-type cytochromes reducible in the presence of succinate are present in A. campestris mitochondria. At 25 degrees C, the difference spectrum reduced in the presence of antimycin minus oxidized shows the two beta-type cytochrome have a common, asymetric alpha-band at 560 nm which splits into two peaks at 559 and 562 nm at liquid nitrogen temperature (-196 degrees C).