The diagnosis of bovine tuberculosis is based on the intradermal tuberculin test. Therefore in eradication campaigns HCSM or PPD tuberculins with high potency and maximal specificity per cattle dose are needed. In quality control and standardization assays the PPD tuberculins have the advantage that the protein content can serve as a basis for calculating the degree of dilution to be used for batches of the concentrated tuberculin, but this chemical estimation does not measure the biological activity of the tuberculo-proteins. Potency control of tuberculins necessitates a bio-assay method, which according to WHO report 384 should be performed under the conditions and in the animal species in which the tuberculin will be used in practice. This potency testing is performed in M. bovis sensitized cattle and guinea pigs, but the bio-assays in both species encounter several problems which must be overcome in order to obtain reliable and consistent results. At the C.V.I. numerous bio-assays have been carried out with various tuberculins and it is the aim of this report to demonstrate that so far the best correlation was obtained between tuberculous cattle and guinea pigs infected with a minimal dose of live M. bovis bacilli, whilst the results with heat killed M. bovis or BCG sensitized guinea pigs were evidently less reliable.