1. A comparative study has been made of the cellular reactions induced by phosphatides from five strains of acid-fast bacilli. Each of these reactions is characterized principally by epithelioid cells and giant cells.

2. The phosphatides are first phagocytized by young connective tissue cells or monocytes. The lipoid is then dispersed into fine particles with the formation of classical epithelioid cells.

3. A comparison has been made of the reactions induced by heat-killed and defatted tubercle bacilli with those induced by tuberculophosphatide.

4. Further studies have been made to determine whether or not the phosphatide causes sensitization to tuberculin. It does not do so.

5. The life cycle of the epithelioid cell has been observed in all its stages.

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