1. Mononuclear exudative cells, obtained from tuberculous guinea pigs by the intrapleural injection of parowax, exhibited characteristic sensitivity to the toxic action of tuberculin when tested in tissue culture.
2. Experiments with these cells, practically free of body fluids, show conclusively that sensitivity to tuberculin is an inherent characteristic of mesenchymal cells from tuberculous animals.
3. Fibroblastic growths which developed from mononuclear exudative cells derived from a tuberculous animal showed persistence of sensitivity to the toxic action of tuberculin on repeated transplantations over a prolonged period in vitro.