1. Quantitative data for both homologous and heterologous precipitin reactions of human, hog, beef, and sheep thyroglobulins show that these reactions have the same mechanism as other instances of the precipitin reaction and may be expressed quantitatively by the same equations derived from the law of mass action.
2. It is shown that all of the added antigen is precipitated in the region of antibody excess and in the equivalence zone, so that in these portions of the reaction range the composition of the specific precipitate may be calculated from the nitrogen precipitated and the amount of antigen nitrogen added.
3. The thyroglobulin-antibody reaction is characterized by low antibody N to antigen N ratios, as would be expected with an antigen of high molecular weight. Molecular ratios varying from 60:1 to 1:1 were calculated for the extremes of the reaction range, indicating a very large number of immunologically reactive groupings on the thyroglobulin molecule.
4. Failure of thyroxine or diiodotyrosine to inhibit specific precipitation was confirmed, but it is shown that this need not mean that these substances do not occur in thyroglobulin, as has been claimed.