No significant difference was found between the biological activity of PPD-preparations derived from M. tuberculosis and M. bovis, although the latter was more potent. In potency tests performed in guinea-pigs, the use of difference mycobacteria as sensitizing agents had no influence on the result of the assay. We could not demonstrate a significantly greater potency of homologous versus heterologous PPD. Evaluation of the 95 p.c. fiducial limits established in the course of variance analysis suggested a greater reliability of reading at 48 than at 24 hrs after allergen application. It also seems possible that the results of the potency assays of PPDs derived from different mycobacterium strains are greatly influenced by the level of the working dilutions employed.