Nucleoside-specific antisera that cross-react with denatured DNA can be obtained by immunization with nucleoside-protein conjugates prepared by a procedure developed in this laboratory (1). Such antisera, and other nucleic acid-reactive antisera, have already been used to investigate some aspects of the structure and function of nucleic acids (cf. 2, 3). Other potential applications of antisera specific for major or minor bases and for di- and tri-nucleotides include immunochemical detection and quantification of bases, possible immunochemical sequence analyses, and immunochemical fractionation of nucleic acids. Immunochemical studies of nucleic acids have been hampered by the limited application of the relatively simple and sensitive radioimmunochemical techniques. In most studies in which these techniques have been employed, the nucleic acids used were labeled in vivo by laborious and frequently difficult procedures (4, 5). Moreover, such methods do not consistently yield nucleic acids with high specific activity.