Summary As compared with soluble bovine serum albumin, the use of the heat-denatured protein as an immunizing antigen in rabbits resulted both in a larger number of responding animals and in markedly higher serum antibody titers. In measuring these titers, it was found that the lower the serum titer, the higher was the percentage of the nonprecipitating antibody in relation to the precipitating variety. Evidently, the appearance of nonprecipitating antibody preceded that of precipitating antibody.