Antisera to several Mycobacterium species (M. bovis, M. avium, M. kansasii and M. smegmatis), capable of reacting with homologous antigen diluted not below 1:16000 in the ELISA test, have been obtained. These antisera have been found to cross-react with other Mycobacterium species. Specific antisera to M. bovis, M. avium, M. kansasii and M. smegmatis have been obtained by the multiple exhaustion method on Mycobacterium antigens giving the maximum cross reaction and used for the isolation of antibodies to these Mycobacterium species. In ELISA exhausted antisera produce reactions in the dilution 1:500. Antisera (antibodies) to M. bovis react with the determinant of the antigen with a molecular weight of 25-26 kD, antisera (antibodies) to M. kansasii react with the determinant of the antigen with a molecular weight of 72 kD, those to M. smegmatis react with the determinant of the antigen with a molecular weight of 38 kD and those to M. avium, with the determinant of the antigen with a molecular weight of 16-18 kD.