A description is presented of a rapid and efficient method for large-scale preparation of ribulosebiphosphate carboxylase (EC 220.127.116.11) from the green alga, Chlamydomonas reinhardtii, and of the separation into two purified subunits. The purification of the enzyme was accomplished essentially by (NH4)2SO4 fractionation and sucrose-density gradient centrifugation, while the separation into subunits has been done by chromatography on sodium dodecylsulfate-hydroxyapatite or by gel filtration on Sephadex G-150 in the presence of guanidine-HCl. The molecular weights of the subunits, as determined by sodium dodecylsulfate-polyacrylamide gel electrophoresis, were 55.0·103 and 16.5·103. The pI of the enzyme was found to be 6.25, while that of the subunits was in the range 6.0-7.0. No amino-terminal amino acid could be detected in either of the subunits. Amino acid composition of the whole enzyme and of the separated subunits were determined and compared to those from other plant species.