The specific activity of ribulose‐1,5‐bisphosphate carboxylase‐oxygenase (Rubisco, EC 4.1.1.39) was measured from the crude extracts of five C₃ plants consisting of wheat (Triticum aestivum L. cv. Maris Mink), spinach (Spinacia oleracea L.), pea (Pisum sativum L. cv. Greenfeast), pumpkin (Cucurbita pepo L. cv. Jättiläismeloni) and Ceratodon purpureus (Hedw.) Brid., and two C₄ plants, maize (Zea mays L. ETA F₁) and sugar sorghum [Sorghum saccharatum (L. emend, L.) Moench]. The amount of Rubisco in the crude extracts was estimated by polyacrylamide gel electro‐phoresis with the Coomassie Brilliant Blue staining procedure. The amounts of the dye bound to the purified Rubisco of different higher plants were similar. The method gave a linear response for both purified enzyme and crude extracts, and the results agreed with those observed by immunochemical methods. The addition of positive effectors such as inorganic phosphate was necessary to obtain maximal activity in the crude extracts of all the studied plants except in that of maize. No significant differences in the specific carboxylase activity at 25°C were found between the C₃ and C₄ plants.