: Two 125I-labelled hormone preparations (synthetic 44–68(Tyr)hPTH bearing a tyrosin at position No. 44 and 1–84 bPTH) were used to characterize antiserum G III for PTH radioimmunoassay. PTH fragments were used for displacement of either tracer and included 1–34 hPTH, 28–48 bPTH, 25–48 bPTH, 28–54 hPTH, 32–43 hPTH, 42–55(Tyr)hPTH (tyrosin at position No. 42), 1–65 bPTH fragments and intact 1–84 hPTH. Standard curves of the 44–68 hPTH, 1–65 bPTH and the 1–84 hPTH run in parallel to each other independent of the tracer used for the assay. When using the 44–68(Tyr)hPTH tracer, half-maximal displacement (1/2 max) was achieved at 24 pg/tube for 44–68 hPTH, 450 pg/tube for 1–65 bPTH and at 1300 pg/tube for 1–84 hPTH. When carrying out the assay with the 1–84 bPTH tracer, the values for 1/2 max were 11.5 pg/tube for 44–68 hPTH, 135 pg/tube for 1–65 bPTH and 370 pg/tube for 1–84 hPTH. Gel filtration of plasma and peritoneal dialysate from one patient with terminal renal insufficiency and secondary hyperparathyroidism indicate that due to the high affinity of the antibody for the mid-C-regional hPTH, this assay system is appropriate for the discrimination of intact PTH and large mid-C-regional fragments, that include most of the 44–68 hPTH sequence.