The immunological heterogeneity of calcitonin (CT) was examined in fractions separated by gel filtration (Bio Gel P-30) from plasma and urine samples of two patients with medullary carcinoma of the thyroid obtained after calcium infusion and after thyroidectomy. Five distinct forms of immunoreactive CT were observed by gel filtration of plasma before treatment; one was eluted with the monomer CT and the other four were of higher molecular weight. The concentrations of all five forms increased during calcium infusion (4 mg/kg, 10 min) and decreased after the infusion, accompanied by a rapid change in concentration of the monomer form. After thyroidectomy, the level of the monomer form decreased faster than the levels of other forms. The main form in the urine was eluted in the same portion as a bigger form (IIIb) in the plasma, not with the monomer form, after thyroidectomy. One antiserum (antiserum I) gave a higher estimation for the IIIb form and the predominant form in the urine than another antiserum (antiserum II). These findings suggest the identity of these two forms. Blood CT in various forms, including the monomer, may be converted to form IIIb in the kidney and excreted in that form.