Animals infected with mycobacteria are allergic to the protein contained in PPD tuberculin and develop characteristic delayed type hypersensitivity react- ions when exposed to tuberculoproteins in tuberculin. If tuberculin is deposited in the skin (intradermally), a local reaction characterized by inflammation and swelling usually is elicited in infected animals, whereas healthy animals fail to develop such responses at the injection site (Thoen, 1988). The comparative tuberculin test in which two or more tuberculins are injected simultaneously offers advantages in differentiating infection with M. bovis from various non-specific infections (Francis et al., 1978). The tuberculin positive cattle show also changes in the serum protein constituents (Moses et al., 1975 and Gatner et al., 1982). The aim of this study was to follow up the delayed hypersensitivity responses in steers infected with a variety of mycobacterial species and the changes in the immune response determined by immunoglobulin levels in their sera.