Summary (1) The solid medium described is a satisfactory medium for the growth of the bacillus of Johne's disease. (2) Inoculation of cultures on the surface of the liquid medium described produces a good growth in the vast majority of cases. (3) Johnin prepared from the broth cultures as described has proved itself to possess qualities rendering its use as a diagnostic agent for Johne's disease of considerable value. (4) The double intradermal Johnin test does not interfere with the double intradermal tuberculin test simultaneously applied. (5) The double intradermal Johnin test produces no reaction in a normal beast or in an animal suffering from tuberculosis. (6) Post-mortem examinations carried out on animals tested, whether reactors or non-reactors, have supported the results of the tests previously applied. (7) Individual sensitiveness of cows to the Johnin test varies to a much greater extent than in the case of tuberculin and tuberculosis.