The results of Loeb, Loeb and Kunitz, and Northrop and Kunitz (Jour. Gen. Physiol. 3, 667 (1920), 5, 693 (1922) and 11, 481 (1927) ) are analysed, to determine the concentrations of gelatin ions in Donnan equilibria with HC1. By using the membrane potentials to calculate the H-ion concentrations, the concentrations of gelatin ions in the presence and absence of NaCl are determined, and it is shown that the addition of NaCl increases the ionisation of the gelatin. This is explained as due to the formation of complex salts between protein and NaCl. Such formation is evidence that the protein near its isoelectric point behaves as a Zwitterion rather than a classical neutral molecule. The solubility of globulins in neutral salt solutions may be due to the same phenomenon. The combination of gelatin with CuCl2 and CaCl2 shows a similar behavior, the gelatin acting as though having a combining power of between 2.5 and 3.5.