The functional and prognostic significance of lymphocytic infiltration of breast carcinomas has remained unclear. Using primary cultures we have demonstrated that lymphocytes could stimulate the growth of breast cancer epithelium in about half of the cases tested. The growth stimulation was subsequently shown to be strongly correlated with expression of MHC class I by the tumour cells. Furthermore, preliminary data suggest that carcinomas with a mixed population of MHC class I-positive and -negative cells were associated with a higher incidence of lymph node metastases and increased relapse rate compared with tumours that were homogeneously MHC class I-positive or -negative. The interactions between breast cancer and the immune system are clearly complex and the results suggest that the nature of these interactions can be some extent be determined by the level and pattern of MHC class I expression by the tumour cells.