1. A pigment can be extracted from Blepharisma undulans by heat treatment of a concentrated suspension of the deeply pigmented animals.

2. In the presence of this pigment, various colorless protozoans are sensitized to light and killed if exposed long enough. The protozoans show a differential sensitivity, some being much more sensitive than others.

3. Bleached colorless blepharismas are not sensitized to their own pigment even after prolonged exposure in the most concentrated solutions available.

4. Blepharisma is also less sensitive than any of the protozoans tested to such photodynamic dyes as rose bengal.

5. The pigment is not extracted from wet blepharismas by non-polar solvents, but is readily extracted into such polar organic solvents as the alcohols.

6. When the alcohol extract is dried, the amorphous residue is readily soluble in a variety of organic solvents, but not in the most non-polar.

7. The pigment is highly stable in alcohol extract and has been kept in the dark for years.

8. The pigment is bleached by light in a photooxidation.

9. Absorption maxima are found at wave lengths 5800, 5400, 4800, and 3300 A, the latter being the largest. Similar peaks are found in alcohol and water solutions, although the heights are not exactly the same. In both alcoholic and aqueous solutions pH had an effect on the absorption spectrum. Heat has little effect but illumination with intense visible light or exposure to ultraviolet light bleaches the pigment with decreases in the characteristic peaks.

10. Preliminary absorption column experiments indicate a single pigment in the alcohol extract.

11. Experiments on the migration of zoopurpurin in agar and gelatin gels indicate that it diffuses at about the same rate as eosin and is therefore probably not a large molecule.

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