The current study was undertaken on 25 cases of thyroid medullary carcinoma to compare the diagnostic value of calcitonin with other peptides including PDN-21, the C-terminal flanking peptide of human calcitonin within the calcitonin precursor, and calcitonin gene-related peptide, CGRP. Antiserum raised to chromogranin, an acidic protein of 68,000 daltons, was also used to compare its diagnostic value as a general marker for neuroendocrine neoplasia with neuron-specific enolase (NSE) and Grimelius' argyrophil silver staining. Immunocytochemistry was performed using the peroxidase-antiperoxidase method at the light microscopic level and the immunogold staining procedure at the ultrastructural level. All tumors were reactive to calcitonin and CGRP antisera, whereas PDN-21 was present in 23 cases. It was also found that these peptides were colocalized in the majority of C-cells. The intensity and specificity of CGRP and PDN-21 immunoreaction was comparable to and in some cases even better than that obtained with calcitonin antiserum. In the majority of tumors, somatostatin and bombesin immunoreactivity was either absent, weak, or variable in intensity and distribution. The current study thus demonstrates that together with calcitonin, PDN and, in particular, CGRP antisera may be applied to corroborate immunocytochemical diagnosis in medullary carcinoma of the thyroid. With regard to general neuroendocrine markers, Grimelius' and chromogranin provided the most consistent results. NSE isoenzyme immunoreactivity, on the other hand, was more variable, probably reflecting the metabolic state of the tumor cells.