In two medullary carcinomas of the thyroid gland two types of secretory granules were found electron microscopically in the cytoplasm of the tumour cells. The sizes of the granules in one case ranged 103-345 nm in diameter; they were round in shape, and they co-existed in the same tumour cell. They could not, therefore, be distinctively subdivided into two types. In another case, secretory granules in the cytoplasm closely resemble EC granule in morphology. Using the protein A-colloidal gold (PAG) technique the content of secretory granules could be identified as calcitonin irrespective of their sizes or morphology. Immunoreactivity at the ultrastructural level was fairly well preserved even in the osmium-fixed tumour cells. The labelling index, expressed as a mean number of gold particles per unit square area of the secretory granule, was higher in the non-osmium-fixed tumour cells than in the osmium-fixed. Non-osmium-fixed tumour cells embedded either in epoxy or methacryl resin were almost equally labelled with gold particles. The result indicates that the PAG method is practicable to demonstrate the ultrastructural localization of calcitonin even in the osmium-fixed, epoxy resin embedded material.