Haemolytic factor, the protein responsible for the haemolysis of red blood cells, was isolated from bull seminal vesicle fluid. The purified protein has a mol. wt of 80,000. Haemolytic factor dissociates in acidic urea buffer into sub-units with a mol. wt approximately ten times lower. Easy transitions in the sub-unit arrangement observed under various conditions are probably the reason for changes in electrophoretic mobility and in haemolytic activity.