Publisher Summary “Haemolysin” is a generic term referring to protein toxins capable of lysing erythrocytes. The secretion of a haemolysin into the external medium by Escherichia coli (E. coli) is presented. Subsequently, both chromosomally encoded human and plasmid encoded animal uropathogenic E. coli isolates secrete haemolysin. Several determinants are subcloned from human and animal isolates including those from Proteus sp. and Actinobacillus pleuropneumoniae. The haemolysins belong to a large family of RTX toxins (repeat in toxin) that possess multiple glycine rich repeats near the C-terminus. The chapter discusses the genetic regulation and function of the E. coli haemolysin secretion system. The in vitro and in vivo role of haemolysin in pathogenesis is equally instructive, demonstrating the co-ordinate regulation of the hly operon with other pathogenic determinants and the in vitro and in vivo action of the toxin upon different cell lines. Secretion of the large 110 kDa haemolytic toxin (HlyA) by E. coli, proceeds by the so-called “Type-I secretion pathway” in Gram negative bacteria. This secretion mechanism requires the products of the four contiguous genes of the hly operon, hly C, A, B, and D.