The ultimate problem underlying many applications of the precipitin test, whether it be the determination of biological relationships, the identification of blood stains and other animal traces, or the detection of adulteration in food, is the recognition of the homologous protein (antigen) and its separation from closely allied heterologous proteins. Recognising that the antiserum is the main source of the precipitate in a precipitin reaction and having regard to the exact quantitative relations of antiserum, antigen and precipitate we have been able to arrange methods for the differentiation of proteins of closely related species and, we believe, to render more accurate the diagnosis of the source of individual proteins. To take a crucial instance, by means of an antiserum prepared with hen egg-white we have been able clearly to distinguish solutions of hen egg-white from all other avian egg-whites tested, including those of the duck, quail, partridge, pheasant and ostrich. So far as we know, Nuttall and Graham Smith alone have previously been successful in differentiating homologous and heterologous avian egg-albumens, and their methods appear to be more cumbersome than ours. Incidentally we have found that our results are not only consistent inter se but consistent also with the interpretation of the precipitin reaction which our previous observations had led us to adopt.