After kaolin treatment of fetal rabbit serum, 7S antibody titers were reduced more than 19S titers. This reduction was less when the kaolin treatment was performed at pH 9.0 than when it was performed at pH 7.3. A modification of the kaolin treatment of sera for use in the hemagglutination-inhibiting antibody titration, in which the hemagglutination reaction is performed at a neutral pH, is recommended. The advantage of the modified method is that adsorption of immunoglobulins to kaolin is minimized when serum is treated at a lower dilution with pH 9.0 kaolin, followed by reduction of the pH of the supernatant fluid to neutrality with a „serum adjusting diluent.” When the serum was diluted with physiological saline before kaolin treatment, a great decrease in serum immunoglobulin concentrations occurred. This decrease was found to be less in the modified kaolin treatment than in the conventional pH 7.3 kaolin treatment.
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