Cytochrome c (Cyt c) has been shown to translocate from mitochondria to the cytoplasm of cells early in apoptosis. In this study sandwich ELISAs for Cyt c were used to determine if Cyt c is ultimately released from apoptotic and necrotic cells. Gel-filtration and cation-exchange chromatographies, in conjunction with immunoreactivity in ELISA, and Western blotting were employed to examine the integrity of the released Cyt c. The results show that Cyt c is released from both apoptotic and necrotic cells in an intact, monomeric form. The release of Cyt c from apoptotic splenocytes began within 2 h following apoptotic insult, while Cyt c was immediately released following induction of necrosis by heat shock. These findings may be relevant to understanding how Cyt c becomes a target for antibody production in some patients with systemic autoimmune diseases.