Antibacterial activity against Escherichia coli was found in the cytoplasmic extracts of rabbit polymorphonuclear leucocytes. The lack of antibody activity was determined by agglutination, passive haemagglutination, bactericidal reaction in the presence of complement, bactericidal plaque technique and immunoelectrophoresis. Thermostability of the extract, higher activity in a sodium citrate buffer than in veronal buffer containing Ca+ + and Mg+ + and negative haemolysis give evidence against the participation of complement in this type of antibacterial activity. Inhibition of the activity following absorption with different bacterial suspensions is interpreted as involving electrostatic bonds between active cationic substances and the negative charge of the bacterial surfaces. In addition, inhibition of the antibacterial activity was proved in the presence of both native and inactivated piglet and rabbit whole sera and albumin, which suggests that there is no independent action of these cytoplasmic substances in serum in vivo.