Summary: An hypothesis to account for the large changes in the antibody-antigen ratios observed in a number of precipitating systems has been presented. Although the hypothesis was derived from an analysis of precipitin and inactivation data obtained with tobacco mosaic virus (TMV), it may find application to other immune systems as well. The basic assumption of the hypothesis is that all antigenic valences are not combined throughout the entire region of antibody excess, but rather that the number of such combinations decreases towards equivalence. In general the fraction of antigenic sites remaining uncombined at equivalence may become larger for antigens of increasing size. The inactivation experiments suggest that the fraction of sites combined with antibody decreases exponentially with increasing antigen concentration. When this exponential function (which is a measure of the changing “effective antigen valence”) is incorporated into the Heidelberger-Kendall equation, a general relationship is obtained, capable of describing a wider variety of data. The equation presented is . The implications and significance of the hypothesis were discussed.