1. A quantitative theory of the precipitin reaction based on the laws of classical chemistry has been tested on an azoprotein-antiprotein system and found to apply. 2. With its aid relationships may be deduced which permit the calculation of the behavior of an antidye serum over its entire range after a few quantitative chemical analyses have been made for antigen and antibody in the precipitate. 3. An empirical relation is also presented which further reduces the number of analyses necessary. 4. A study of supernatants in the inhibition zone has shown that the entire amount of dissolved antigen-antibody compound present is precipitated when supernatants are analyzed for antigen by the precipitin method.