Up to 25% of the total protein in green leaves consists of a single molecular species, the enzyme ribulose bisphosphate carboxylase/oxygenase (rubisco). In C3plants rubisco molecules are found densely packed within chloroplast stroma at concentrations up to 300 mg/mL. The oligomeric protein (MW 550,000) is composed of eight large and eight small subunits which combine to form a compact, nearly spherical molecule. Rubisco is easily purified from leaf extracts in the laboratory but, as yet, there is no simple, economical method for isolating large (kilogram) quantities of the plant enzyme. The protein appears to have potential as an ingredient in animal and/or human foods. Highly purified rubisco is a tasteless, odorless, white powder with a nutritive value reported to be equal to or superior to that of other food proteins. Rubisco also possesses some desirable functional properties which might enable food processors to successfully incorporate the protein into a number of different food systems. However, much further research is needed before rubisco becomes an acceptable substitute for other animal or plant proteins.