IT has been shown by Frappier and Guerault1,2, and Frappier, Guerault and De Repentigny3, that the antigen of Haemophilus pertussis which protects mice against intracerebral infection could be washed off the bacteria by saline (0.85 per cent sodium chloride). As this protective effect in the mouse is accepted as an indicator of the efficiency of vaccines in protecting children against whooping-cough it would appear that saline washings afford a ready means of obtaining the immunizing antigen of the organism free from the bulk of the bacterial cell. The washings were obtained from bacteria grown in a fluid medium, they were practically devoid of toxicity and stimulated agglutinin production in rabbits.