Breast carcinomas, either positive or negative with the Grimelius' silver procedure, benign fibroadenomas, duct papillomas, and areas of histologically normal breast tissue were tested immunocytochemically with the mouse monoclonal antibody LK2H10 directed against human chromogranin. This is regarded as a general stain for polypeptide-hormone-producing cells and tumors. In 3 of the 9 cases of argyrophilic carcinoma, but in none of 12 ductal infiltrating carcinomas, chromogranin-positive cells were found: the number of reactive cells was very low in 1 case, while in the other 2 carcinomas about 50% of the argyrophilic cells appeared stained. In areas of histologically normal breast tissue, rare argyrophilic chromogranin-positive cells were detected. This study is the first reported evidence concerning the presence of endocrinelike cells probably belonging to the diffuse neuroendocrine system in the normal mammary parenchyma. Our data are consistent with the endocrine nature of at least some of the breast argyrophilic carcinomas.