Quantitative studies on the inhibition by haptenes of the antigen-antibody precipitate have been made on three systems: 1. Pneumococcus type II polysaccharide and type II antiserum, with sodium euxanthate as inhibitor. 2. Partially hydrolysed cherry gum and type II pneumococcus antiserum, with sodium euxanthate as inhibitor. 3. Atoxyl-azo-egg albumin and atoxyl-azo-globulin antiserum, with atoxyl as inhibitor. In all three systems the inhibition by a given concentration of haptene varied very little over a wide range of antigen/antibody ratios. While the amount of precipitate depends mainly on the total amounts of antigen and antibody, varying little with dilution, the degree of inhibition depends on the concentration and not the total amount of haptene. In system 1 there was only partial inhibition even with high concentrations of haptene. The concentration of haptene required to give half the maximum effect was about sixteen times as great as in system 2. In system 3, the ratio of antibody to azo-protein in the precipitate is lowered by the presence of haptene. There are wide variations in sera from different rabbits in the concentration of haptene required to give the same degree of inhibition. The bearing of these results is discussed on the nature of the action between antigen and antibody.