The antigenic relationships of the major prolamin storage proteins and low-molecular-weight alcohol-soluble proteins of barley, wheat and rye and the prolamins of oats are studied using polyclonal antisera raised against purified protein (or groups of proteins) and quantitative laser nephelometry. In addition, the specificity of a previously characterized antiserum against C hordein of barley is studied using purified peptides and chemically modified C hordein. The antigenic relationships of these divers proteins are complex, but in many cases can be explained in terms of known amino acid sequence relationships. In other cases, antigenic relationships exist between proteins with little known amino acid sequence homology. Several lines of evidence, including the studies of the C hordein antiserum, indicate that the repetitive sequences present in the major groups of prolamins are highly immunogenic, and may be the location of some conformational epitopes. Studies with the low-molecular-weight alcohol-soluble proteins (chloroform/methanol-soluble proteins and low-molecular-weight prolamins) show that they are related immunologically to each other and to some of the major groups of prolamins. They also have close immunochemical relationships with avenins of oats.