Abstract
A study of the precipitability by the appropriate antisera of 34 different haptens, containing from one to six reactive groups, leads to the conclusion that the possibility of framework ("lattice") formation is neither necessary nor sufficient for specific precipitation, but that instead precipitation depends upon the reduction, by mutual neutralization of polar groups of antibody and antigen (or hapten) and mechanical blocking off of polar groups of closely neighboring molecules of antibody, of the solubility of the complex below the point at which it can remain in solution. The decisive factors appear to be the number of polar groups of the antigen (hapten) left free, and the distance separating the different reactive groups, which determines the amount of steric hindrance exerted by one antibody molecule on another. No hypothesis is offered as to how these primary insoluble aggregates unite with each other to produce the larger aggregates which are finally observed.