Antiserum against a 36 kD polypeptide (a subunit of arachin) that occurs in poor blanching peanuts was prepared. This antiserum was of high titer but was not monospecific for the homologous 36 kD anitgen; it showed cross-reactivity with other peanut proteins. The antiserum was adsorbed by the whole seed protein fraction of a good blanching cultivar to render it monospecific. The adsorbed antiserum could accurately differentiate the protein extracts from good blanching and poor blanching peanuts when tested by the enzyme linked immounosorbent assay (ELISA) and by immunoblotting. Time course studies of protein deposition during seed development revealed that the 36 kD polypeptide was present from the early stages of embryogenesis. In addition, it was present in all parts of the cotyledon and not limited to the surface of seed. Although the function of the 36 kD polypeptide is not known, our results indicate that its presence can be detected uising immunological assays.