Methods for the isolation and purification of B. tularense antigens which involve ether treatment, salt and alcohol precipitation, differential centrifugation, and dialysis have been presented. Immunogenic activity was tested in mice, and it was found that the degree of protection was proportional to the precipitation titer of the antigen, within certain limits, and was not affected by the amount of non-antigenic material present in the various preparations tested. The antigenic material released by either treatment has been characterized by differential centrifugation as a polydisperse system of particles.