Introduction. Interest in the equilibrium of precipitation reactions is now about fifteen years old, and dates from certain observations made in 1902 by Linossier and Lemoine, and by Eisenberg. Linossier and Lemoine (9) mixed antigen and precipitin in the test-tube, and, after removing the resulting precipitate, again tested the supernatant fluid for the presence of each factor. For example, horse serum, and the serum of a rabbit immunized thereto, were mixed in various proportions; the supernatant fluids in each tube were then divided into two equal portions, and to the one was added horse serum, and to the other the immune rabbit serum. If precipitation resulted in the former, it indicated the presence of remnants of precipitin; if in the latter, it showed that a fraction of the original horse serum still remained in the supernatant fluid. As a matter of fact, Eisenberg (6, 7) found that both substances could practically always be demonstrated.