1. A sensitive interferometric method of observing antigen-antibody complexes formed by diffusion of the reactants through gels in suitable diffusion cells is described.

2. Bands of antigen-antibody complex, if not too dense, are observed as peaks in the interference fringes, the areas under the peaks being proportional to the quantities present in each band.

3. The method may be made quantitative by measurement of the areas under the peaks of bands in which precipitation has not progressed too far, and by dissolving the bands of dense precipitate by diffusion of a suitable reagent to give peaks in the fringes.

4. An equation expressing the quantity of precipitate present in a band has been derived.

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