The genus Proteus belongs to the tribe of Proteae in the family of Enterobacteriaceae, and consists of five species: P. mirabilis, P. vulgaris, P. morganii, P. penneri and P. myxofaciens. They are distinguished from the rest of Enterobacteriaceae by their ability to deaminate phenylalanine and tryptophane. They hydrolyze urea and gelatin and fail to ferment lactose, mannose, dulcitol and malonate; and do not form lysine and arginine decarboxylase or beta-galactosidase [1]. Colonies produce distinct "burned chocolate" odor and frequently show the characteristics of swarming motility on solid media. P. mirabilis, P. vulgaris and P. morganii are widely recognized human pathogens. They have been isolated from urinary tract infections, wounds, ear, and nosocomial bacteremic infections, often in immuncompromised patients [2-6]. P. myxofaciens has no clinical interest to this time. P. penneri as species nova was nominated by the recommendation of Hickman and co-workers [7]. Formerly it was recognized as P. vulgaris biogroup 1 or indole negative P. vulgaris [8, 9]. Although it has been less commonly isolated from clinical samples than the other three human pathogenic Proteus species, it has nevertheless been connected with infections of the urinary tract, wounds and has been isolated from the feces of both healthy and diarrheic individuals [10-12]. Potential virulence factors responsible for virulence of Proteae are: IgA protease, urease, type3 fimbriae associated with MR/K haemagglutinins of at least two antigenic types, endotoxin, swarming motility and HlyA and/or HpmA type hemolysins [for review see ref. 13]. In the followings we give a survey of accumulated concepts about the position and characteristics of HlyA type alpha-hemolysins both in general and with emphasis on virulence functions in the tribe of Proteae.