Summary: The effect of varied NaCl concentrations on the reaction of chicken antisera with serum protein antigens was studied by turbidimetric and quantitative nitrogen techniques. The results of the two methods paralleled each other closely. Increasing the NaCl concentration up to that range in which there was some salting out of chicken serum proteins (14–18%) resulted in marked increases of precipitation with the maximum occurring at progressively higher antigen concentrations. The reaction was solely between specific antigen and antibody, and the effect of increasing the salt content was to cause a more complete precipitation of this antigen and antibody. Thus determinations of maximum antibody content could not be made at salt concentrations of less than about 8%. The precipitation also became more rapid with increases of salt concentration and reacting antibody. The significance of the above results was discussed.