Calcitonin was measured in the plasma of 13 patients with medullary throid carcinoma and one patient with a calcitonin-producing islet cell carcinoma of the pancreas. The hormone was measured by the simultaneous application of two calcitonin antisera which had differing specificity. Calcitonin measurements were also made of gel filtration (Bio Gel P30) fractions of plasma samples. Although the two antisera were of comparable sensitivity for the measurement of human calcitonin standards, they reacted differently with the calcitonin in the plasma samples. One antiserum (LD-1) gave consistently higher estimations of hormone concentration in the plasma of patients with medullary thyroid carcinoma than the other (LD-26). By contrast, the concentration of calcitonin from the islet cell carcinoma of the pancreas was higher in the LD-26 assay. Multiple gel filtration peaks of immunoreactive calcitonin activity were present in both thyroidal and nonthyroidal calcitonin. Furthermore, the two different antisera identified differing immunochemical peaks of calcitonin activity for agiven plasma sample. These findings demonstrate the presence of immunochemical heterogeneity in plasma calcitonin and suggest the presence of immunological differences between thyroidal and nonthyroidal calcitonin.