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SUMMARY (1) Calcitonin preparations from acetone-dried thyroid were administered to rats by various routes. (2) Intravenous administration, especially by infusion, produced a much greater fall in plasma calcium than s.c. or i.p. injection. (3) The log dose-effect curves after i.v. injection or infusion showed no evidence of non-linearity over a 100-fold dose range and had highly significant slopes. (4) The potency ratio of two preparations was estimated by means of a (2+2) assay design using both i.v. infusion and single i.v. injection. There was satisfactory agreement. (5) The i.v. injection method is recommended for the routine assay of calcitonin. A simple assay schedule is given in the Appendix. (6) A unit of calcitonin activity is defined in terms of a standard preparation.

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