1. Leucocytes ingest quartz particles more readily than carbon in acid solutions, and carbon more readily than quartz in alkaline solutions.

2. In the presence of acacia carbon is always preferred to quartz even in acid solutions.

3. Manganese dioxide particles are ingested by leucocytes with extraordinary rapidity as compared with manganese silicate or quartz.

4. Leucocytes are not attracted toward carbon or quartz particles but manganese dioxide exerts a distinct attraction for them.

5. Spores of Penicillium are ingested more readily than quartz.

6. Very small quartz particles, 1 micron in diameter, are not ingested as readily as larger particles of the same material. This result being contrary to the predictions of surface tension indicates that some other factor is involved in the ingestion of these small particles.

7. Measurements of the carbon electrode potentials and the cataphoretic charges on the particles have failed to supply an explanation for the varying relative rates of ingestion of carbon and quartz with varying hydrogen ion concentration.

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