We describe a sequential double-antibody assay for measuring C-terminal parathyrin in serum with commercially available reagents. Intact bovine hormone, used as a working standard, is iodinated by conventional Chloramine T procedures. The antibody affinity is characteristic of high affinity binding (1.4 to 1.6 x 10(10) L/mol of intact parathyrin). The antibody also cross reacts with a C-terminal parathyrin fragment (amino acids 53-84) but not with a synthetic N-terminal parathyrin fragment (amino acids 1-34). The assay thus also measures both a C-terminal fragment of parathyrin and the intact hormone. The detection limit (250 ng/L; 500 int. units/L) is below the reference interval for healthy adults (430-1860 ng/L; 860-3720 int. units/L). Several commonly recognized problems with iodinated parathyrin are eliminated and accuracy and precision of the procedures for standard preparation and calibration are improved. Overall CV (between-run imprecision) is 10-17%. Analytical recovery is 80-90%, and C-terminal parathyrin measured in fresh sera and in sera stored for seven days at 30 degrees C is equivalent.