1. A new quantitative method of measuring phagocytosis of solid particles is described. 2. A method of calculating the chances of collision between leucocytes and quartz particles of different sizes is developed. 3. The speed with which three suspensions of different sized quartz particles should be ingested by leucocytes is predicted from the calculated chances of collision, and the prediction is verified experimentally. 4. The formula for the chances of collision is also verified by varying the speed of rotation of the tubes in which the phagocytic mixtures are incubated.