Bovine parathyroid hormone (PTH) was purified up to the Sephadex-stage. The biological activity of the preparations was about 2000 U/mg solid material. Rabbits and guinea-pigs were immunized with crude and Sephadex-purified material. Antisera elicited in two guinea-pigs proved to be useful for further studies. Purification of 125I-labelled bovine PTH was carried out by adsorption to and desorption from antibodies or by means of microfine silicate. The purified labelled hormone showed excellent binding to the antibodies. Maximum sensitivity of the assay system of 10 pg per incubation medium could be achieved by a pre-incubation method. The selectivity of the system was studied.