Properties of ribulose-P ₂-carboxylases from wheat, maize, tobacco, spinach and pea and one photosynthetic bacterium (Rhodospirillum rubrum) were compared. Electrophoresis showed no differences in mass of the large subunit polypeptides of the higher plant carboxylases. The small subunit polypeptides from spinach, pea and tobacco carboxylases had similar molecular weights of about 14000. This value is smaller than that of the small subunit of wheat and larger than that of the small subunit of maize (carboxylase). The purified carboxylases differed in their content of a rapidly activating form of the enzyme and in their sensitivity to Mg ²⁺. The differences were not easily fitted into discrete categories so that no clear classification was possible on the basis of activation characteristics. Relative activities of the various enzymes as carboxylases and oxygenases were quantified as specificity factors. The carboxylase from R. rubrum, was much less effective as a carboxylase compared to an oxygenase than carboxylases from the higher plants. Specificity factors for the higher plant enzymes were between 77 and 104. The highest values were recorded for the carboxylase of wheat and the lowest for carboxylases of maize and tobacco. The differences in specificity factors between carboxylases from wheat, maize and tobacco are discussed in relation to differences in amino acid sequences of the large subunit polypeptide. Four differences in residues within the active site may be relevant with respect to the observed differences in enzyme properties. Mild trypsinolysis very rapidly decreased the activity of carboxylases from maize, wheat and spinach but pea, barley and tobacco carboxylases were inactivated more slowly and the enzyme from R. rubrum was not affected at all.