Upon treatment with p-mercuribenzoate under alkaline conditions, spinach leaf ribulose-1,5-diphosphate carboxylase [EC 4.1.1.39] dissociates into its constituent subunits. A partial reassociation of the dissociated components into the native enzyme molecule (18S), with concomitant recovery of the enzyme activity, can be achieved by the addition of excess β-mercaptoethanol to the unfractionated subunits. The reversible nature of the dissociation was studied by. means of sedimentation experiments in an analytical ultracentrifuge and also by sucrose density gradient centrifuga-tion and polyacrylamide gel electrophoresis. The extent of the reassociation of the dissociated components into the native enzyme was 30–35%, recovering a specific activity about 80% that of the intact native enzyme. Among other thiol compounds tested, dithiothreitol was superior to β-mercaptoethanol, about 37% recovery of the original enzyme activity being attained, while reduced glutathione was least effective. The partial reconstitution of the original carboxylase molecule from the separated constituent subunits was established by observing the activating effect (50%) of the smaller subunit on the enzyme activity conveyed by the catalytic subunit, As, in concomitant reformation of the original native enzyme molecule, as determined by polyacrylamide gel electrophoresis.