An immunocytochemical method was used to screen smears obtained at primary surgery from multiple bone-marrow sites in 110 patients with breast cancer; at this time other techniques did not reveal metastases. Tumour cells were detected in the bone-marrow of 31 (28%) patients. The number of cells detected ranged from 1 to greater than 500; none was detected in conventionally stained smears. Patients in whom conventional criteria indicated a very poor prognosis seemed more likely to have such micrometastases. A further follow-up period is required to determine whether patients with micrometastases relapse earlier than those in whom no tumour cells are demonstrable.