Biological assays carried out in the Republic of Ireland in naturally sensitized (M. bovis infected) tuberculous cattle have shown that Rotterdam and Weybridge bovine PPD tuberculins issued for routine use in 1981 and 1982 were equipotent with an estimated potency of about 24,000 Community Tuberculin Units (C.T.U.) per mg of PPD. However, previously it was shown that Weybridge bovine PPD produced in 1978 had only a potency of about 4,000 C.T.U./mg in M. bovis infected cattle. The clinical significance of the latter finding is seen in the results of field trials carried out in the Republic of Ireland in 1978 to determine the relative diagnostic abilities of Weybridge and Rotterdam, routine issue bovine PPD tuberculins. These trials involved the tuberculin testing and slaughter of naturally sensitized cattle. Post-mortem and laboratory examination confirmed that 68 of these animals were infected with M. bovis. It was concluded that the injection dose for the diagnosis of bovine tuberculosis in cattle should not contain less than 2,000 C.T.U. and that the use of an injection of double this dosage was desirable in known M. bovis infected herds.