1. The rates of ingestion of quartz and carbon particles by leucocytes, when both are in suspension in serum, was compared with the availability of the two particles as predicted from the calculated chances of collision with the leucocytes, and it was shown that carbon is ingested about 4 times as readily as quartz.
2. The greater ease of ingestion of carbon was verified by a new method of measuring phagocytosis, described as the film method in which the cells ingest particles as they creep about on a slide.
3. The relative rates of ingestion of carbon and quartz depend upon the condition of the cells, the difference increasing as the phagocytic activity of the cells decreases.
4. Sponge cells also ingest carbon about 3 times as readily as quartz.
5. The hypothesis is suggested that the cause of the more rapid ingestion of carbon may be identical with the cause of the greater instability of the carbon suspensions.
6. An inorganic analogy to this selective phagocytic action is offered.
7. The application to opsonins and agglutinins is discussed.