A combination of immunocytochemistry (ICC) and in-situ hybridization (ISH) applied to formalin-fixed tissue sections was used to analyse the differential expression of calcitonin and CGRP genes, at both peptide and mRNA levels, in normal and neoplastic human thyroid C cells. Calcitonin peptide was readily detectable in normal C cells but its abundance in the neoplastic C cells of medullary thyroid carcinoma (MTC) was reduced in correlation with the degree of tumour differentiation. Conversely, the content of calcitonin mRNA was higher in MTC than in normal C cells and was not significantly related to tumour differentiation. Both the peptide and mRNA of CGRP were present at much lower levels than those of calcitonin in normal C cells but were increased in neoplastic C cells. We conclude that neoplasia of thyroid C cells is associated with (i) an increase in the content of CGRP mRNA and peptide relative to that of calcitonin, consistent with a defect in control of transcript processing, and (ii) a decrease in the ratio of calcitonin peptide to mRNA abundance relative to the normal, suggesting a defect in synthesis or storage of the peptide. ISH analysis of calcitonin mRNA may therefore be a very valuable addition to ICC analysis of the peptide as a diagnostic and prognostic marker for MTC.