Despite the presence of a lymphocytic infiltrate in solid cancers, the failure for tumour growth to be contained suggests an inadequate immune response to the tumour. Poor cytotoxicity exerted by tumour-infiltrating lymphocytes (TILs) against tumour cells in vitro, combined with continued tumour growth in vivo, suggests deficiencies in TIL function or numbers. Various theories have been postulated to explain how tumour cells may escape immunosurveillance and control. One of the many hypotheses is the failure of production of cytokines, which are necessary for T cells to mediate their function. Thus, the expression of cytokine mRNA in human breast tumour sections was investigated by reverse transcriptase polymerase chain reaction (RT-PCR) with cytokine-specific primers. A relatively consistent finding was detection of interleukin (IL) 10 mRNA among the tumours. No IL-2 and little IL-4 mRNA was detected in the tumours. IL-6 and IL-10 mRNA was detected in only one and two of the normal breast tissues respectively. IL-2, IL-4 and tumour necrosis factor (TNF)-alpha mRNA was not detected in any of the normal breast tissues. The reduced function of TILs may be related to IL-10, which has known inhibitory effects on T-cell activation.