Abstract The binding of cytochrome c to the cytochrome bc 1 complex of bovine heart mitochondria was studied. Cytochrome c derivatives, arylazido-labeled at lysine 13 or lysine 22, were prepared and their properties as electron acceptors from the bc 1 complex were measured. Mixtures of bc 1 complex with cytochrome c derivatives were illuminated with ultraviolet light and afterwards subjected to polyacrylamide gel electrophoresis. The gels were analysed using dualwavelength scanning at 280 minus 300 and 400 minus 430 nm. It was found that illumination with ultraviolet light in the presence of the lysine 13 derivative produced a diminution of the polypeptide of the bc 1 complex having molecular weight 30 000 (band IV) and formation of a new polypeptide composed of band IV and cytochrome c. Band IV was identified as cytochrome c 1, and it was concluded that this hemoprotein interacts with cytochrome c and contains its binding site in complex III of the mitochondrial respiratory chain. Illumination of the bc 1 complex in presence of the lysine 22 derivative did not produce changes of the polypeptide pattern.