The brand of yeast extract used for medium production is critical for the cultivation of large numbers of Corynebacterium pseudotuberculosis. The agglutination test was used to measure the antibody response which followed the inoculation of various inactivated C. pseudotubermlosis vaccines . The best results followed two subcutaneous injections of 5 ml vaccine prepared by inactivation with 0,5% formalin and standardized so that each dose contained a total of 0,025 ml packed cells. The addition of adjuvants to the vaccine had a negligible effect on the antibody response, but increasing the interval between the primary and secondary injections from 2 or 4 to 6 weeks resulted in a higher level of agglutinating antibodies. The antibody titres, however, returned to pre-immunization levels within 3 to 4 months. Freshly prepared vaccine was very toxic, apparently due to incomplete toxoiding of the exotoxin, but older vaccine was shown to be quite safe. A method was devised whereby chronic pulmonary abscesses could be established in experimental animals. Exposure of immunized sheep to artificial infection showed that they were well protected against subacute infection and death but that they were not able to restrict the development of abscesses effectively. The possible reasons for the disappointing results are discussed in terms of the mechanisms of immunity which may be involved.