Described herein is a process for isolating the enzymatic protein ribulose 1,5-diphosphate carboxylase from the green leaves of plants. In the process, which is particularly suited to obtaining the protein from tobacco, the leaves are ground or otherwise pulverized in the presence of a reducing agent, followed by heating the resulting pulp to about 50 DEG C. A liquid portion containing the desired protein is separated from the pulp and then cooled to cause the crystallization of the ribulose 1,5-diphosphate carboxylase. After separation of the crystalline material, the supernatant is acidified to yield lower molecular weight proteins.
