Antibodies to synthetic human calcitonin (hCT) were developed in rabbits, goats and mice. The free peptide (32 amino-acid residues, Mwt. 3418) was administered together with adjuvant, and the effect of various immunization procedures, as well as of different dose-levels, was evaluated comparatively. Synthetic hCT was found to be a good immunogen for the three animal species examined. The relative importance of various structural parts of the hCT molecule with regard to immunological specificity was determined by reference to the inhibition of the specific binding of 125I-hCT to antibodies by peptide fragments of hCT. All the antisera studied were directed to structural and/or conformational properties of the 11-28 or 11-32 amino acid sequence of hCT. Six different antisera from rabbits and goats were selected for radioimmunological assay of hCT on the basis of their inhibitory dose50-values and immunological specificity. To improve the sensitivity of the radioimmunoassay (RIA), we studied the preparation of radioiodinated hCT and assessed various parameters determining the sensitivity of the assay. Despite all the efforts, CT in human plasma from healthy subjects could not be determined with certainty. The difficulties encountered in the determination of normal levels of circulating CT are discussed in terms of the sensitivity of RIA and non-specific interference of serum factors with RIA.