Converse, J. L. (U. S. Army Chemical Corps, Frederick, Md.), M. W. Castleberry, A. R. Besemer, and E. M. Snyder. Immunization of mice against coccidioidomycosis. J. Bacteriol. 84:46-52. 1962-The observed lower virulence of Coccidioides immitis arthrospores of the M-11 strain compared with the Cash strain was ascribed to the lower ability of the M-11 spores to invade the lungs and brain. Endospores of strain M-11, grown in synthetic medium, were equally as virulent by the intraperitoneal route as arthrospores of strain Cash. Immunization of mice with viable strain M-11 arthrospores protected approximately 50% of the animals against challenge with strain Cash arthrospores. This was evidenced by lower mortality over a 5-month period and lack of gross pathological lesions in killed animals. Although fewer lesions resulted after the use of a viable vaccine, higher survival was obtained with formaldehyde-killed arthrospores. Either preparation tended to confine the infection to the peritoneal cavity.