We have found large quantities of immunoreactive carboxyl-terminal fragments of human parathyroid hormone )hPTH) in a previously discarded fraction [the 7.5% trichloroacetic acid (TCA)supernate] generated during extraction of intact hPTH from hyperfunctioning parathyroid tissue by the urea-TCA procedure. It is well established that serum RIAs directed toward the carboxyl-terminal region of hPTH are superior to those directed toward the amino-terminal region in the differential diagnosis of patients with suspected chronic parathyroid dysfunction. However, antisera that react with the carboxyl-terminal region of hPTH are not yet available for general use for these assays because of a lack of suitable hPTH immunogens. We immunized seven guinea pigs and two goats with the desalted 7.5% TCA supernate (containing about 2% carboxyl-terminal hPTH fragments); three of the guinea pigs and one goat produced high affinity antisera with predominant specificity for the carboxyl-terminal region of PTH. One of the guinea pig antisera had affinity for hPTH equal to that of our laboratory's best antiserum (GP1M) used in diagnostic RIAs for serum PTH. The use of this byproduct fraction as an immunogen should permit a large scale immunization program in large animals to provide standardized, species-and sequence-specific antisera potentially useful in RIAs for diagnosis of parathyroid disease.