Control of bovine tuberculosis (TB) in the UK has largely been carried out through the application of the test and slaughter policy. The test performed in the UK is the intradermal comparative cervical tuberculin test to distinguish between animals infected with Mycobacterium bovis from those exposed to environmental mycobacteria. This policy brought the incidence of bovine TB from ~30% in the 1960s to just below 0.1% in the 1970s. However, since the 1980s there has been an increase in the incidence of bovine TB which is thought to be due, at least in part, to the existence of a M. bovis wildlife reservoir. This increase indicates that the application of the test and slaughter policy, in the presence of a wildlife reservoir, may not be adequate for the control of bovine TB and that improved control measures are required. Vaccination has been proposed as a possible control measure. Vaccination with M. bovis bacillus Calmette-Guerin (BCG) has been shown to confer a variable degree of protection. One of the factors thought to be responsible for this variability in efficacy is the pre-exposure of individuals to environmental mycobacteria. If BCG vaccination is to be considered as a tool for the control of bovine TB, it is necessary to better understand the effect of exposure to environmental mycobacteria prior to BCG vaccination. Similarly, improvement of diagnostic tools requires a better understanding of the mechanisms by which the currently available tuberculin test works. In this work data are presented indicating that inoculation of cattle with M. avium, a surrogate for environmental mycobacteria, does compromise the diagnosis of bovine TB, but also affords a degree of protection against M. bovis. Data on the cellular basis of the immune response elicited in mycobacteria-sensitised cattle by mycobacteria purified protein derivative (PPD) are also presented.