<p>A comparative study of the properties of cytochrome <i>c</i> in its interaction with phosphatidic acid is carried out. Similar conformational changes in the active center of cytochrome <i>c</i> occurring during its interaction with both cardiolipin and phosphatidic acid are demonstrated using the methods of spectrofluorometry. The presence of similar magnitudes of peroxidase activity of cytochrome <i>c</i> in its interaction with both cardiolipin and phosphatidic acid was corroborated by luminol-dependent chemiluminescence, and the use of the EPR method enabled the detection formation of phenoxyl radical of etoposide in both complexes. It can be concluded that both phosphatidic acid and cardiolipin effectively alter the structure of the active center of cytochrome <i>c</i> and contribute to the enhancement of its peroxidase activity. The obtained results may indicate a very probable participation of both cardiolipin and phosphatidic acid in the development of cytochrome <i>c</i>–mediated proapoptotic processes in mitochondria.</p>