The question of the mechanism of "prozone" creation was considered from the point of view of the concentrations of free and semi-blocked bivalent antibodies in the mixture of these antibodies with monovalent antigen. Using the so called "coordinates of dilution", suggested by the author earlier, it was possible to calculate the relationships between the concentrations of either free or semi-blocked bivalent antibodies and the dilution of the antigen-antibody mixture. It was shown that dilution of antigen-antibody mixture leads to an increase of the concentration of free bivalent antibodies and simultaneous sharp decrease of the concentration of semi-blocked antibodies. It is suggested, that such a relationship is quite enough for the creation of prozone effect in reactions, when only bivalent antibodies are active and semi-blocked antibodies compete with free antibodies, providing inhibition of the reaction.