A field trial on a country-wide basis was undertaken to compare the specificity for bovine tuberculosis of single and comparative tuberculin tests in cattle using either Weybridge human or Weybridge bovine PPD. The tests were made on 10,305 cattle in 179 herds distributed throughout all regions of England, Scotland and Wales. Results showed that a comparative tuberculin test using avian PPD with either human or bovine PPD had a much higher efficiency than a single injection of mammalian tuberculin in the neck of cattle, and confirmed that a comparative test is still essential in the British environment. Weybridge bovine PPD gave significantly better discrimination between tuberculous and non-tuberculous cattle than Weybridge human PPD when used together with avian PPD in a comparative tuberculin test. The diameter of induration gave an absolute measure of the extent of oedema, if present, and induration diameter used in conjunction with skin thickening increased the sensitivity and specificity of the test. Rules of interpretation were developed and are presented for an intradermal comparative tuberculin test in cattle using Weybridge avian and bovine PPDs.