A double immunohistochemical technique, incorporating MIB1 and CD3, was used to identify proliferating T cells in paraffin wax sections of normal tonsil and B cell lymphomas. The number of double stained T cells as a percentage of the total T cells was then determined. In normal tonsil and follicular lymphoma the follicle centre and T cell zones were counted independently. In normal tonsil very few T cells in the follicle centre expressed MIB1. Proliferating T cells were concentrated in the T cell zones. The same pattern was observed in follicular lymphoma. In contrast, the percentage of T cells expressing MIB1 was higher in mucosa associated lymphoid tissue type lymphoma and lymphocytic lymphoma, suggesting that T cell activation occurs in these tumours. The highest percentage of MIB1 positive T cells was observed in high grade lymphoma. This suggests that transformation to high grade lymphoma is associated with an increase in T cell activation.