Native Glycoprotein II (Phaseolin, G-II), the major 7S storage protein from <i>Phaseolus vulgaris</i> seeds, var. 'Processor' is known to be resistant to <i>in vitro</i> proteolysis by most endopeptidases. On sequential treatments with pepsin and a mixture of trypsin and chymotrypsin, the sub-unit polypeptides of G-II were split midchain. The fragments produced however, retained reactivity with the antibody raised against native G-II quantitatively. When measured by rocket immunoelectrophoresis, the extent of <i>in vitro</i> degradation of G-II by these endopeptidases was negligible. This procedure was used for monitoring the <i>in vivo</i> or <i>in vitro</i> degradation of G-II by gut enzymes other than trypsin or chymotrypsin. Diets containing 10% of a highly purified G-II preparation, did not support growth of rats adequately. Faecal N outputs were elevated and the true N digestibility based on Kjeldhal estimation was only 37%. In contrast, the true GII-N digestibility, based on immunological estimations, was high. It is suggested that G-II and/or its limited breakdown fragments (by trypsin or chymotrypsin) are stimulants of endogenous N secretion in the small intestine. The higher extent of the degradation of G-II in the small intestine of rats <i>in vivo</i> than that obtained by pure endopeptidases <i>in vitro</i> suggested the presence in this tissue of other enzymes capable to act upon and modify the structure of G-II, prior to the action of trypsin and chymotrypsin. These other modifying proteolytic enzymes render the G-II molecule more negatively charged and more susceptible to the subsequent action of trypsin and chymotrypsin. It is suggested that protease content and the ratio of the concentration of the GII-modifying protease(s) to that of trypsin and chymotrypsin may vary appreciably along the small intestine. Accordingly, the dependence of the degradation of G-II <i>in vivo</i> on the competition between all the enzymes capable of attacking it during its passage through the gut may explain the variability of GII breakdown <i>in vivo</i>.