IT is well known that the ‘delayed’ or ‘infectious’ type of sensitivity is induced in a normal organism, after contact with living or killed tubercle bacilli, but it is not passively transferred by means of serum or extracts of organs from ‘allergic’ animals. This type of sensitivity is also induced in vitro by a mixture of ‘purified wax’ and tuberculoprotoin and in vivo by transferring leucocytes from ‘allergic’ donors. The intensity of the sensitivity proved to depend on the percentage of mononuclear cells present in the leucocytic suspension. By disrupting the ‘sensitized’ cells, a component resembling an antibody of the plasma fraction called ‘IV–10’ fr. or an α-globulin was released, which was found to induce a ‘delayed’ sensitivity.