The majority of tissue specimens submitted for the isolation ofMycobacterium boviscontain contaminating microorganisms and therefore require selective decontamination before bacteriological examination. The purpose of this study was to identify the preferred decontaminant amongst four commonly used reagents. The four decontaminants used in the study were 1-hexadecylpyridinium chloride, sodium hydroxide, benzalkonium chloride and oxalic acid. A comparison was made of the toxicity of the four decontaminants forM. bovisand their ability to control contamination. Used at the recommended concentrations, all reagents showed a significant degree of toxicity. The toxicity of the decontaminants for three field strains and one laboratory strain ofM. boviswere similar, but a second laboratory strain, AN.5, was more susceptible. It was also observed that as the concentration of each reagent decreased an abrupt change from control to lack of control of contaminating micro-organisms occurred. Hexadecylpyridinium chloride was found to be the best all-round reagent because at concentrations that effectively controlled contamination this reagent was the least toxic toM. bovis.A 1orotocol for handling specimens based on an assessment of the risk of contamination is recommended.