Immunocytochemical staining with monoclonal antibody NCRC 11 of formalin fixed paraffin embedded tumour tissue has been studied in 444 cases of primary breast cancer with a minimum follow period of 6 years. The relationship between extent of staining, assessed on a four point scale, and patient survival has been confirmed. There are significant relationships between staining and both histological grade and oestrogen receptor status. No association has been shown between staining and lymph node stage or tumour size. Simplification of staining assessment by modification to two staining groups still allows significant separation of patients into prognostic groups and incorporation into an existing prognostic index.