Summary: A procedure has been described whereby the quantity of precipitate resulting from the action of precipitin and antigen can be measured. At one point in titrations, as usually practiced, the greatest quantity of precipitate is formed and this we have called the zone of maximum precipitation. With this as a basis we have been able to show that the quantity of precipitate will vary proportionately with change in quantity of either antigen or immune serum. A linear type of reaction has been demonstrated. In addition similar linear relationships have been shown in the determinations of antigen and antibody in the supernatant after precipitation has taken place. In cow serum and its precipitin we have not been able to show that in a given series either antigen or antibody is completely utilized. Where a single antigen, such as egg albumin, was employed with its precipitin, there occurred within a narrow zone complete binding of both antigen and antibody, while in either direction one or the other was still present in active form.