By simultaneous dilution of the antigen (AG) and the antibody (AB) used in an immune precipitation reaction, at the equivalence AG/AB ratio, it was found that the association constant, Ka, increased from 1.7 X 10(7) 1/M at the highest concn of AG and AB, to 6.5 X 10(11) 1/M at 100-fold dilution of both. An important (but not the only) reason for this increase is taken to be the fact that the AB solution was polyclonal.