Immunoglobulins IgY, extracted from yolks of hens immunized against several proteins and natural mixtures of proteins were examined by the Ouchterloney gel diffusion and Laurell immuno-electrophoresis techniques. The molecular weights of the proteins ranged from 8 x 10(6) to less than 2 x 10(4). The hens produced IgY antibodies readily when injected with the high molecular antigens > 150 000 but did not react as well to antigens of relatively low molecular weight < 30 000. Optimal precipitin reactions were obtained with low molecular antigens when the agarose gel contained 1.5 M NaCl. Salt concentration (0.15-1.5 M NaCl) has no effect on the precipitin reaction in gels on the systems of high molecular antigens and their IgY antibodies apart from a displacement of the precipitin line. From the relative positions of the precipitin lines to the wells containing the reactants it would appear that the IgY formed dimers when the concentration of NaCl is 1.6 M and remain as monomers in gels containing 0.15 M NaCl. An explanation of the salt effect is offered.