Messenger RNA from calcitonin-secreting rat medullary thyroid carcinoma lines directs the synthesis of a putative precursor of calcitonin when translated in wheat embryo lysate. The major translation product directed by mRNA isolated from a rat tumor with a calcitonin content representing greater than 2% of the total protein mass has an apparent molecular weight of 17,500 when analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Immunoprecipitations of the cell-free translation products demonstrate that the 17,500-dalton protein has immunologic determinants which are specifically recognized by antisera to calcitonin. In addition, the relative prominence of this protein is significantly diminished when mRNA from lines with low calcitonin content is used to direct cell-free protein synthesis. This evidence suggests that the initial translation product of calcitonin mRNA is a 17,500-dalton species which must undergo subsequent processing to generate the secreted polypeptide.