Summary: The results of a quantitative study of the inhibition reaction, using dyes of different degrees of resemblance to the haptens concerned, are not incompatible with the conception that the injection of a single “complete” protein coupled with diazotized arsanilic acid leads to the production of two different antihaptens, one corresponding more closely to the tyrosin-diazo-arsanilic acid group, the other to the histidin-diazo-arsanilic acid group. In the case of the “deficient” protein, gelatin, the coupled protein seems to give rise to only one of these antihaptens.