Horse heart cytochrome c can be treated with cyanogen bromide under conditions resulting in division of the molecule into a heme peptide of 65 residues and a peptide of 39 residues, corresponding to segments 1-65 and 66-104 of the parent protein, respectively. Upon mixture of the two peptides in aqueous solution, a complex can be formed that is slowly converted to a product with properties nearly indistinguishable from those of the initial heme protein. Evidence is presented that in this slow phase of the reaction the fragments are rejoined into a continuous chain through restoration of the peptide bond linking residues 65 and 66.