Samples from medullary, intraductal, tubular and adenocystic breast carcinomas were studied ultrastructurally, with emphasis on the patterns of myoepithelial differentiation and basal lamina deposition. Myoepithelial cells, while prominent in the intraductal and adenocystic carcinomas, were rarely found in the medullary neoplasms and appeared absent in the tubular neoplasms. Parallel to the above, basal laminae were most abundant and evenly deposited in the intraductal and adenocystic tumors, but were infrequent in the medullary tumors and seemingly absent in the tubular carcinomas. Well-defined myoepithelial cells and retention of the capability to synthesize basal lamina are evidence of differentiation in neoplastic cell populations. Thus, their presence in intraductal and adenocystic carcinomas (generally associated with a good prognosis) is not surprising. However, their scarcity or absence in the similarly favorable group of medullary and tubular carcinomas suggests that other factors may also influence the invasive and metastasizing ability of breast carcinomas.