Ribulose bisphosphate carboxylase, a key enzyme in the photosynthetic carboxylation process, has been localized through an indirect immunofluorescent technique in the guard cells of some of the 41 species of plants examined. This sample includes 17 families of both dicotyledons and monocotyledons, one gymnosperm, and one pteridophyte. Plants were selected to represent all of the three major photosynthetic categories, namely C(3), C(4), and Crassulacean acid metabolism. Antibodies raised against tobacco (Nicotiana tabacum L.) ribulose bisphosphate carboxylase were used for this immunofluorescent study. A good degree of fluorescence was observed in the guard cells of seven out of 21 species exhibiting Crassulacean acid metabolism. C(3) plants exhibited a very low degree (almost negligible) of fluorescence, while the C(4) species did not exhibit any fluorescence.