Cytolysin refers to the substance secreted by microorganisms, plants or animals that is specifically toxic to individual cells, in many cases causing their dissolution through lysis. Cytolysins that have a specific action for certain cells are named accordingly. For instance, the cytolysins responsible for the destruction of red blood cells, thereby liberating hemoglobins, are named hemolysins, and so on. Cytolysins may be involved in immunity as well as in venoms.
Hemolysin is also used by certain bacteria, such as Listeria monocytogenes, to disrupt the phagosome membrane of macrophages and escape into the cytoplasm of the cell.