SUMMARY: A two-site assay has been developed for bovine PTH. This technique involves reaction of the antigen with two antibody molecules with the purpose of increasing specificity. In practice the hormone was extracted from plasma samples on to plastic tubes coated with antibody specific for PTH (1–34). Uptake was then measured using 125I-labelled antibodies specific for PTH (53–84). In this way a sensitive assay was obtained for PTH (1–84) which did not recognize molecular fragments. This technique was used in conjunction with immunoradiometric assays specific for either NH2-terminal (1–34) or CO2H-terminal (53–84) molecular fragments to study the clearance of infused PTH in the cow. Preliminary results support the hypothesis that the intact molecule is rapidly degraded in the peripheral circulation with the preferential disappearance of an NH 2-terminal fragment. Studies on endogenous secretion during calcium and EDTA infusions indicated that there was little intact hormone present at the times when CO2H-terminal immunoreactivity was readily measured.