Ultrastructural immunoreactivity for chromogranin with the LK2H10 antibody was examined in nine cases of breast carcinoma and one example of normal resting breast. These tissues were selected for study on the basis of argyrophilia or LK2H10 immunostaining by light microscopy. In two cases, positive reactivity with the chromogranin antibody was seen in breast neuroendocrine-like dense core granules. In a further six cases chromogranin reactivity was not seen in similar granules that showed the neuroendocrine properties of ultrastructural argyrophilia and uranaffinity. Inability to exhibit the full complement of neuroendocrine characteristics in breast carcinomas contrasts with the facility to demonstrate them in tissues of usual neuroendocrine differentiation. Furthermore, in two mucoid carcinomas and one example of normal resting breast ultrastructural cytoplasmic LK2H10 reactivity was evident, which was not localized to dense core granules, although these were present in two of the cases. Our findings demonstrate the heterogeneity of breast dense core granules and discourage acceptance of such characteristics as evidence of histogenesis of these carcinomas from a neuroendocrine cell type in breast tissue.