Thirty-eight infiltrating ductal carcinomas, nine infiltrating lobular carcinomas, two tubular carcinomas and one papillary carcinoma were studied by light microscopy, immunocytochemistry and electron microscopy. Seventeen cases showed immunoreactivity for NSE. Immunostaining for different peptide-hormones was observed in 12 of these 17 cases and in none of the 10 NSE-negative cases used for controls. Scattered cells were positive for gastrin in five cases, pancreatic polypeptide in five, leu-enkephalin in three, sub-P in two, ACTH in one, bombesin in one and beta-endorphin in one case. Four cases revealed immunoreactivity for more than one peptide-hormone. Typical neuroendocrine granules were seen in five cases (all positively stained for NSE). Small, electron dense granules of possible neuroendocrine nature were not found in any of the 33 NSE-negative tumours. Our results confirm that immunoreactivity for NSE is present in a high proportion of breast carcinomas, but that neuroendocrine differentiation cannot be proved to be present in all these cases.