Summary: The specific antigen found in intestines of sheep infected with paratuberculosis (Johne's disease) was investigated. The antigen was precipitated by 50 per cent saturation with ammonium sulfate, and by 2 per cent trichloroacetic acid. It does not diffuse through a dialyzing membrane. Extracts have been obtained containing the antigen, that give a negative Molisch test for carbohydrate but they give a positive Biuret test. The antigen is insoluble in ether and thermostable. It is therefore concluded that the antigen is either a protein or closely associated with a protein. In 31 out of 33 cases tested the antigen was found to be masked in the infected mucosa. Practical methods of demasking the antigen are described although the nature of the masking is not understood.