A rapid method is described for the preparation of up to 500 milligrams of pure ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBP carboxylase) from 250 grams of field-grown soybean leaves. Leaves were extracted in 20 millimolar phosphate (pH 6.9) at 4 degrees C, containing 4% (w/v) polyvinylpolypyrrolidone, 10 micromolar leupeptin, 1 millimolar phenylmethyl sulfonylfluoride, 1 millimolar diethyldithiocarbamate, 5 millimolar MgCl(2), 1 millimolar dithiothreitol, 0.2 millimolar ethylene-diaminetetraacetic acid, 50 millimolar 2-mercaptoethanol. The extract was incubated in the presence of 5 millimolar ATP at 58 degrees C for 9 minutes, then centrifuged and concentrated. Sucrose gradient centrifugation into 8 to 28% (w/v) sucrose on a vertical rotor for 2.5 hours yielded pure enzyme with a specific activity of 1.1 to 1.3 micromoles per minute per milligram protein at pH 8.0, 25 degrees C. Soybean plants of the same line grown (at 400 microeinsteins per square meter per second) in growth chambers yielded enzyme with a specific activity of 0.6 to 0.7 micromoles per minute per milligram protein. During prolonged purification procedures a proteolytic degradation of RuBP carboxylase caused complete loss of catalytic activity. Without destroying the quaternary structure of the enzyme, a 3 kilodalton peptide was removed from all large subunits before further breakdown (removal of a 5 kilodalton peptide) occurred. Catalytic competence of the enzyme was abolished with the loss of the first (3 kilodalton) peptide.