We have performed an immunohistochemical study of immunoblastic T cell lymphoma and enteropathy-associated T cell lymphoma for alpha-1 anti-trypsin and CD30. Cytoplasmic staining for alpha 1 anti-trypsin is present in the malignant cells in both types of T cell neoplasm which also express CD30, a marker of lymphoid activation. Peripheral blood T lymphocytes on stimulation with mitogen also show granular cytoplasmic expression of alpha 1 anti-trypsin. Time course studies show that this parallels the expression of CD30. Alpha 1 anti-trypsin expression appears therefore to be associated with activation in T cells. Further studies of sub-fractionated T lymphocytes in vitro suggest that the expression of alpha 1 anti-trypsin on activation is not restricted to an individual lymphocyte subset.