SUMMARY: An antigen containing the phyto-oestrogen genistein (5,7,4′-trihydroxyisoflavone) was prepared by coupling genistein-2-carboxylic acid to a copolymer of tyrosine, glutamic acid and lysine. Immunization of rabbits with this antigen resulted in the production of antibodies to the carrier and to the hapten. Genistein-specific antibodies were isolated by means of an immunoadsorbent containing genistein linked to a heterologous carrier; these reacted with genistein-containing antigens in the passive cutaneous anaphylaxis, complement fixation and 125I-labelled antigen binding tests. The antibodies failed to cross-react with oestradiol-17β coupled to ovalbumin. The reaction of the genistein—polypeptide conjugate with the purified antibodies was inhibited by genistein and by the closely related oestrogenic isoflavone biochanin-A. No such inhibition was produced by oestradiol-17β, nor were the anti-genistein antibodies capable of binding 14C-labelled oestradiol. The potential use of immunization with isoflavone conjugates for the protection of livestock against phyto-oestrogens is discussed.