Many Vibrio species are pathogenic to humans, and/or marine vertebrates and invertebrates. The pathogenic species produce various virulence factors including enterotoxin, haemolysin, cytotoxin, protease, lipase, phospholipase, siderophore, adhesive factor and/or haemagglutinins. Haemolysin, which is an exotoxin that lyses erythrocyte membranes with the liberation of haemoglobin, is arguably the most widely distributed toxin among pathogenic vibrios and exerts various roles in the infection process. Haemolysins act on erythrocytes membranes thus lysing the cells which leads to the freeing up of the iron-binding proteins namely haemoglobin, transferrin and lactoferrin. This iron can then be picked up by various siderophores, and is subsequently taken up through receptors in the cell membrane. In many cases, the pore-forming activity of haemolysin is not restricted to erythrocytes, but extends to a wide range of other cell types including mast cells, neutrophils, and polymorphonuclear cells, and enhances virulence by causing tissue damage. There are four representative haemolysin families in Vibrio spp., including the TDH (thermostable direct haemolysin) family, the HlyA (El Tor haemolysin) family, the TLH (thermolabile haemolysin) family and the d-VPH (thermostable haemolysin) family. Some haemolysins, for example, TDH of Vibrio parahaemolyticus and HIyA of Vibrio cholerae have been studied extensively, and are closely associated with virulence. However, the role of some haemolysins, e.g. TLH and d-VPH are unclear, and await the outcome of further research. © 2005 The Society for Applied Microbiology.