The liberation of non-protein nitrogen, soluble in 12 per cent. Trichloroacetic acid, from various milk protein fractions by rennin (0,1 fLg rennin NJml) at pH 6�7 and 25�C is examined. 1� 0 per cent. nitrogen is rapidly released from whole casein, 3� 4 per cent. from second-cycle casein-fraction S, and 6� 7 per cent. from K-casein. None is split from other milk protein fractions under the same conditions.