The haemolytic activity of broth culture filtrate ofAcinetobacter calcoaceticusgrown at 22°C has been investigated. The haemolytic activity was enhanced by Mg++ and Mn++ and inhibited by Zn++ and Hg++. Red cells from different species exhibited different haemolytic patterns. The temperature coefficient (Q10°C) was estimated at 1.8, the reaction being rectilinear for 1 hr at 37°C. The haemolytic material was non‐dialysable, sensitive to heat and proteolytic enzymes and antigenic on injections of filtrate into rabbits. The nature of the haemolytic material is briefly discu