Limited proteolysis of ribulose-bisphosphate carboxylase with trypsin or endopeptidase Lys C causes the loss of the carboxylase and oxygenase activities, without disrupting the binding of bisphosphates or the reactions normally associated with activation of the enzyme. Gel electrophoresis of the non-denatured enzyme indicates that the L₈S₉ quaternary structure of large and small subunits is unaffected by this treatment. However, electrophoresis in denaturing conditions shows that the L subunit is smaller as a result of the removal of two short polypeptides, each approx. 1000 Da. The loss of activity is associated with the removal of the second peptide. The amino acid composition of the isolated peptides indicates that both originate from the N-terminus of the L subunit. Over protracted periods of exposure to the proteases a third smaller peptide is also released from the N-terminus of the L subunit. The S subunit is not affected by these treatments. The carboxylase isolated from a number of photosynthetic species exhibits a differential response to this limited proteolysis.