The hydrolysis of 14C-labeled ribulose-1,5-bisphosphate carboxylase (RuBPCase) by two partially purified endoproteinases from senescing barley (Hordeum vulgare v. Numar) leaves is described. The major thiol proteinase, EP1, exhibits biphasic kinetics which appear to be caused by a region of the large subunit of RuBPCase that is highly sensitive to attack by EP1. This proteinase further hydrolyzes both the large and small subunit to smaller peptides. A second proteinase, EP2, appears to convert the small subunit of RuBPCase rapidly to a 13.7-kilodalton fragment during initial stages of hydrolysis and then to degrade both this fragment and the large subunit. The presence of a third endoproteinase, EP3, was discovered when [14C]RuBPCase, which appeared to be homogeneous by sodium dodecyl sulfate polyacrylamide electrophoresis, seemed to undergo very low but significant rates of “autolysis.” The large molecular weight fragments produced by EP3 were different from those of EP1 and EP2.