To obtain a polychlonal antibody having excellent bonding properties and high peculiarity to bile acid, by immunologically sensitizing suitable vertebrate species with a homopolymer which can be formed by polymerizing monomer bile acid shown by a specific formula.

The homopolymer is a polymer or oligomer of bile acid which can be formed by polymerizing monomer bile acid shown by a formula G-X-A where G is bile acid group or derivative, X is a crosslinking group, and A is a polymerizable ethylene unsaturated group. By using the homopolymer, a suitable vertebrate species, e.g. a rabbit or a sheep is immunologically sensitized, and polychlonal antibody having excellent bonding properties and high peculiarity to bile acid can be manufactured. On the basis of the antibody, immunoassay method of bile acid which ensures quick accurate assay of bile acid content in biological matrix can be used.