The effects of cytochrome c and apocytochrome c on the structural properties of various membrane phospholipids in model systems were compared by binding, calorimetric, permeability, 31P n.m.r. and freeze-fracture experiments. Both cytochrome c and apocytochrome c experience strong interactions only with negatively charged phospholipids; apocytochrome c interacted more strongly than cytochrome c. These interactions are primarily electrostatic but also have a hydrophobic character. Cytochrome c as well as apocytochrome c induces changes in the structure of cardiolipin liposomes as is shown by 31P n.m.r. and freeze-fracture electron microscopy. Cytochrome c does not affect the bilayer structure of phosphatidylserine. In contrast, interaction of apocytochrome c with this phospholipid results in changes of the 31P n.m.r. bilayer spectrum of the liposomes and also particles are observed at the fracture faces. The results are discussed in relation to the import of the protein into the mitochondrion.