Semisynthesis is a method of protein engineering that relies primarily on chemical manipulations of peptide fragments derived from natural proteins to produce intermediates for reassembly into defined analogs. In this respect it differs fundamentally from total chemical synthesis, but like it, it has unique capabilities for introducing noncoded amino acids or site-specific labels into the protein product. A favorite subject for semisynthesis is the redox protein cytochrome c: a vital link in the respiratory chain. The tactics that have evolved in the half dozen laboratories so engaged are briefly described. More than 100 semisynthetic analogs of this protein have resulted from this effort and have contributed to our understanding of the way in which the function of cytochrome c is determined by its three-dimensional structure. Questions concerning the folding and stability of the protein, the setting of heme redox potential, control and specificity of interaction with potential redox partners, and the mechanism of electron transfer are considered. A growing synergy between this approach and genetic methods will ensure that protein engineering by semisynthesis continues to increase our knowledge of this and other proteins.