Soybean vegetative storage proteins (S-VSPs) are lysine-rich and, hence, are potentially of high nutritive value for high productive ruminants. Using S-VSPs from wild-type soybean and from transgenic tobacco plants expressing either one of the two S-VSPs subunits (S-VSP alpha or S-VSP beta) or both, we tested their stability in cow rumen fluid under in situ conditions, using SDS-polyacrylamide gel electrophoresis. Proteolysis and degradation pattern of S-VSPs from transgenic tobacco leaves occurred relatively fast compared with that of wild-type (WT) soybean plants. Comparing the two S-VSPs subunits expressed in transgenic plants, we found that S-VSP alpha was degraded much faster than S-VSP beta. The degradation pattern of S-VSPs in transgenic tobacco plants expressing both subunits resembled that of WT soybean. In contrast, the degradation pattern of transgenic tobacco plants expressing a single subunit was different. These finding suggest that the quaternary structure of S-VSPs may be an important factor determining their resistance to rumen degradation. Our results also suggest that the stability to rumen proteolysis of a given protein, when expressed in a transgenic plant, may not always be predictable and has to be verified.