Abstract
The nature of the precipitin reaction of dog intestinal phosphatase with its rabbit antiserum have been investigated with special emphasis on the region of excess antibody. Evidence for the existence in this region of soluble antigen-antibody complexes has been presented and discussed. The use of horse antibodies against rabbit γ-globulin has been demonstrated to be useful for detecting and studying soluble complexes of antigen and antibody. The use of horse antibodies for the quantitative precipitation of phosphatase at levels estimated to be below 1 γ is described.