2. The intravenous injection of large amounts of potent antihorse rabbit serum immediately after an injection of horse serum does not materially accelerate the rate of disappearance of the horse serum from the circulation. While the number of days that circulating antigen was demonstrable in two experiments of this type was less than the average of controls the duration of persistence was within the limits of variation of normal controls.

3. Under the condition of the experiments intravascular union of antigen and antibody is an unimportant factor in the mechanism for disposal of antigen. By exclusion it therefore seems probable that the rate of disappearance of antigen is largely dependent upon cell avidity for the antigen.

4. Rabbits show a wide range of individual variation in the rapidity with which foreign serum is appropriated by the tissue cells, and in their ability to form precipitins. There does not appear to be any close relation between the amount of precipitin set free in the circulation and the rate of disappearance of precipitinogen.

5. The type of interrelation between precipitin and precipitinogen—demonstrable not infrequently in patients with serum disease—in which the precipitinogen diminishes rapidly when the precipitin rises to the crest of its curve is sometimes reproduced in rabbits. The type of curve which has been observed in patients with little or no serum disease in which the precipitinogen persists steadily in the circulation for many weeks and little or no antibody is formed, has not been observed in the present series of rabbits.