A mutation at the rb locus of pea (Pisum sativum L.) alters the shape, reduces the starch content, and increases the lipid and sucrose contents of the seed. These effects are probably all consequences of a reduction of up to 40-fold in the maximum catalytic activity of ADP glucose pyrophosphorylase in the developing embryo of the mutant relative to the wild type. We have investigated how the mutation brings about this reduction in activity. The purified enzyme from mutant embryos has a specific activity about 10-fold lower than that from wild-type embryos, and it is much more sensitive to the effectors inorganic phosphate and 3-phosphoglycerate than the wild-type enzyme. Both wild-type and mutant enzymes consist of polypeptides of around 50 kilodaltons. One of the polypeptides of the purified wild-type enzyme is missing from the mutant enzyme. We deduce that in the wild-type embryo this protein may interact with other subunits to confer a high specific activity and a low susceptibility to effectors on the enzyme.