We investigated the cause of elevated immunoreactive circulating parathyroid hormone (PTH) levels in two females of 41 (case 1) and 39 (case 2) years of age with low/normal serum calcium levels and hypocalcemia, respectively, and, in the latter case, hyperprolactinemia. Serum samples from both patients were fractionated by Sephadex G-100 Superfine chromatography. Fractions were assayed for PTH and prolactin (PRL) by immunoradiometric assays (IRMA) and for immunoglobulin G (IgG) by radial immunodiffusion. Sera from both patients were incubated with protein A and protein G Sepharose, centrifugated and the supernatant was assayed for PTH by IRMA. Sera were also subjected to affinity chromatography with an anti-human-IgG-agarose column. IgG and PTH or PRL were measured in the fractions by radial immunodiffusion and IRMA, respectively. In both cases the majority of serum PTH immunoreactivity eluted in the same fractions of IgG after gel filtration and was precipitated by protein A (89% in patient 1 and 96% in patient 2) and protein G (83% in patient 1 and 100% in patient 2), thus, behaving as IgG. In case 1, 79% of PTH was also retained by an anti-hIgG agarose column. High PRL levels in patient 2 were due to macroprolactinemia since most of PRL eluted as big,big (40%) and big-PRL (45%) after gel filtration. Forty-eight percent of PRL from patient 2 was retained by the anti-hIgG column indicating the presence of an anti-PRL autoantibody. These data suggest that spontaneously occurring anti-PTH autoantibodies must be considered in the differential diagnosis of patients with elevated serum PTH levels.