Proteins of the cytoplasmic membrane of Mycobacterium avium were investigated to identify those which were: (1)intrinsic or extrinsic, (2) attached to the cell wall, (3)surface accessible and (4) excreted. In addition sera containing anti-cytoplasmic membrane proteins were obtained and preliminary purification of the cytoplasmic membrane protein was attempted. The predominating cytoplasmic membrane protein of 31,000 daltons (MCMP) was found to be intrinsic, attached to the cell wall and possibly surface accessible. The MCMP was not excreted, even in media in which the MCMP is not found in the cytoplasmic membrane. Other cytoplasmic membrane proteins were also found to be intrinsic; a few were likely to be extrinsic based upon their separation from the membrane in sucrose gradients. Cytoplasmic membrane proteins of 66, 000, 115, 000 and 129 dalton were surface accessible as judged by I 125-Iodobead labeling. Antisera against the HCMP and other cytoplasmic membrane proteins was obtained and will be useful in further cytoplasmic membrane protein characterization. Acetone precipitation of a cytoplasmic membrane preparation was performed to partially purify the MCMP. The data from this study can be used for the development of serodiagnostic reagents for detecting mycobacterial infection. / Master of Science