We have investigated the proliferation rates of T-cell subsets in colorectal carcinomas using immunohistochemistry. It was found that the tumour-infiltrating T cells in contact with the tumour cells have a significantly higher frequency of proliferation than those in the stroma. In particular, the CD8+ intraepithelial lymphocytes (T-IEL) within the tumours have a significantly higher frequency of proliferation in comparison with CD8+ T cells in the stromal compartment or in any normal mucosal lymphoid tissues. It is possible that the proliferation of the CD8+ T-IEL may be driven by self-antigens expressed on the tumour cells. The proportion of CD3+ CD7- T cells is increased within carcinomas compared with the normal colon, and a population of CD57+ T cells was observed which is absent from the normal colon. It is possible that these phenotypes are acquired in situ due to repeated stimulation of the T cells by tumour antigens. Intact colorectal carcinoma explants were cultured, and the presence of tumour-infiltrating T cells analysed after 3 days of culture in isolation from the systemic compartments. CD3+ T cells were proliferating (at a low rate) within the explants after 3 days of culture, indicating that they may be sustained by factors present in the tumour microenvironment.