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Within the framework of a study on the immunostimulant properties of Brucella abortus and of its extracts, we have investigated the protective activity on infection of mice by Klebsiella pneumoniae and on Charlotte Friend's leukemia, of inactivated Brucella abortus and brucella lysates by ultrasound. The injection of inactivated B 19 R preparations and their lysates three days before infection by klebsiella has allowed to protect mice against a large number of DL50 (100 and 1000 in certain cases). To determine the role of the surface antigen in this type of immunostimulant, the activity of the B 19 R inactivated preparations has also been examined. It appears that the protection obtained by the injection on day J + 3, in relation to the infection, does not depend on the presence of the complete surface antigen, while the long term effect seems to be more dependent on this. The development of Charlotte Friend's leukemia has been favourably influenced by the previous injection of B 19 S or inactivated B 19 R. Immunotherapy on days J + 1 or J + 3 gave encouraging results with B 19 R, but facilitation with B 19 S. These results confirm the immunostimulant activity of brucella which has already been reported by ourselves and by other authors.