Presence of a substance in the Third International Standard of Old Tuberculin that interacts negatively with the biological potency of the preparation.

In the course of experiments designed as part of a biological assay of the Third International Standard of Old Tuberculin (OT) it was noticed that this preparation at the usual concentrations failed to elicit the expected concentration-response in persons on whom the von Pirquet was performed.Further specially designed experiments have shown that this phenomenon is due to the presence in the Third OT Standard of a substance that interferes with, or blocks, the biological response to the tuberculin. However, the substance has not yet been identified nor has a model yet been constructed to explain in detail the nature of the interaction between that substance and the tuberculin.