The monoclonal antibody NCRC-11, which has epithelial membrane antigen (EMA)-like immunoreactivity, was used to identify intracytoplasmic lumina in a series of 105 adenocarcinomas from various sites and in 283 breast carcinomas; 55% of the non-breast carcinomas and all breast carcinomas except one of spindle cell type contained intracytoplasmic lumina. The highest frequency (16.4% of tumour cells) was found in invasive lobular carcinoma of the breast. The use of antibodies with EMA reactivity is advocated in the routine investigation of metastatic and undifferentiated tumours.