Plastid and cytosolic isozymes of ATP:fructose 6-phosphate 1-phosphotransferase (PFK(p) and PFK(c), respectively) have been isolated from leaves and developing endosperm tissues of the castor oil plant (Ricinus communis L). Endosperm PFK(p) has been purified to apparent homogeneity. Polyclonal antibodies raised against one of the four polypeptides associated with potato tuber PFK (molecular mass, 46 kilodaltons) immunoprecipitated developing endosperm and leaf PFK(p), but not PFK(c) isozymes. Western blots, sodium dodecyl sulfate polyacrylamide gel electrophoresis, and analytical gel filtration show that PFK(p) from developing endosperm is a 220 kilodalton homotetramer composed of 57 kilodalton subunits. Kinetic studies of leaf PFK(p) and PFK(c) isozymes reveal both similarities and differences to the characteristics of the respective endosperm isozymes studied previously (WJ Garland, DT Dennis [1980] Arch Biochem Biophys 204: 302-317). The immunological and kinetic data suggest that leaf and developing endosperm PFK(p) are different but structurally related proteins.