The course of the tryptic digestion of caseinogen and gelatin was followed in the 2-bulbed dilatometer which enabled the reaction to be investigated from its very beginning. During the first 30 or 40 min., changes were registered by the dilatometer but not indicated by the detns. of amino-N; thereafter the dilatometric depression was proportional to the liberation of amino N in the case of both proteins. The abnormality occurring in the initial stages of the digestion was closely connected with the process of liquefaction which generally preceded the rapid cleavage of proteins. The dilatometric depression per millimol liberation of amino N was 8.7 mm.3 for gelatin and 10.8 for caseinogen. The constants appeared to be functions of the structure and amino-acid make-up of the proteins.