Germinating rice seeds were fed with [(35)S]methionine and the incorporation of (35)S into beta-amylase demonstrated by quantitative immunoprecipitation using rabbit anti-beta-amylase immunoglobulin G fraction. Separation of the antigen-antibody complex by Na-dodecylsulfate gel electrophoresis and subsequent radioautography clearly showed the radioactive labeling of the beta-amylase molecule. The specific radioactivity of beta-amylase derived from scutellum by immunoprecipitation was significantly greater than that of the endosperm. The results strongly indicate that at the onset of germination of rice seeds beta-amylase is synthesized de novo in the scutellum and that in later stages there occurs activation of an inactive, latent form of the enzyme associated with starch granules in the endosperm. In later stages of germination this activated form of the enzyme becomes dominant.