1. Synthetic detergents of the anionic, cationic and nonionic types result in the rapid and constant formation of Charcot-Leyden crystals from eosinophils.

2. Charcot-Leyden crystals have a negative crystalline birefringence and form penetration twins.

3. The changes taking place in the eosinophil in the formation of Charcot-Leyden crystals under the influence of wetting agents, utilizing phase and polarizing microscopy, are described.

4. In the formation of Charcot-Leyden crystals with wetting agents, the nucleus of the eosinophil lyses with no appreciable effect on the granules.

5. In the formation of Charcot-Leyden crystals with a wetting agent, there is no change in the lipoid cortex of the eosinophil as demonstrated by staining with sudan black B.

6. Charcot-Leyden crystals undergo changes on standing that affect their solubilities.

7. The staining reactions and solubilities of Charcot-Leyden crystals are described.

8. Oxyhemoglobin crystals constantly form from red cells on exposure to Aerosol MA; on two occasions, tyrosine crystals formed from the blood of a patient with chronic myelogenous leukemia.

9. Evidence is offered that Charcot-Leyden crystals are crystalline proteins derived only from the nucleus of the eosinophil.

ACKNOWLEDGMENTS I wish to thank Dr. William B. Ober, Dr. J. J. Englefried, and Lt. E. E. Ozburn, MSC, USN, for assistance in this work.