Lymphocytic infiltration of breast cancer is often associated with a favourable prognosis. Seventy-seven women with operable breast cancer were followed for a minimum of 3 years. Tumours were frozen and sectioned by cryostat before staining with monoclonal antibodies using an immunoperoxide technique for total lymphocytes, helper/inducer, suppressor/cytotoxic, natural killer and B subsets. Lymphocyte density was assessed by light microscopy at x400 and divided at the 50th percentile to less than 30 and greater than or equal to 30 cells per high power field to give scanty and dense lymphocytic infiltration. The helper/suppressor lymphocyte ratio was greater than 1 in 45 patients but without improvement in survival or cancer recurrence. Natural killer and B lymphocytes were rarely seen in tumour sections. Poorly differentiated tumours excited a more marked lymphocytic infiltration (P less than 0.01). Neither the density of lymphocyte infiltration nor the ratio of helper to suppressor lymphocytes correlated with improved short-term survival or recurrence. These data suggest that the immune defence is ineffective in preventing spread from the primary tumour in breast cancer patients.