The major protein component of rice leaves was partially purified by extraction of the leaf proteins and gel filtration of the extracts on Sephadex columns. This protein was characterized by starch-gel electrophoresis as the predominant protein band, and in the purified preparation was the only protein component discernible. The isolated protein preparation is believed to be identical to fraction I protein as studied by others. The fixation of 14C02, using ribose 5-phosphate as substrate in the system, was definitely catalyzed by the isolated protein fraction. The main product of 14CO2 fixation was 3-phosphoglyceric acid. The three enzymatic activities involved in the system, namely, phosphoriboisomerase, phosphoribulokinase, and ribulose 1,5-diphosphate carboxylase, are believed to be associated with the fraction I protein of rice leaves.

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