Metallothioneins (MT) are low molecular weight, metal-binding proteins. The induction of MT synthesis by cytokines, hormones and other cytotoxic agents indicates its role in cellular proliferation and differentiation as well as in cellular defense mechanisms. Previous studies have detected expression of MT in various human tumors by immunohistochemical staining. In certain cases the presence of MT in a tumor may be associated with its resistance against radiation and chemotherapeutic agents. Immunohistochemical staining of MT using a rabbit polyclonal anti-rat liver MT antibody was carried out in eight gastric, two small bowel and one large bowel lymphomas, and in ten control gastrointestinal (gastric, colonic and small bowel) biopsies or excised bowel segments with benign lymphoid infiltrates. The primary antibody against rat liver MT readily cross-reacts with human MT. The neoplastic cells in nine of 11 malignant lymphomas showed weak to intense staining for MT, mostly in cytoplasm. In these cells a few nuclei (less than 5% of all tumor cells) were stained positively for MT. The benign lymphocytes in the gastrointestinal excised specimens and biopsies were mostly negative; four cases showed few positive cells. The giant cells were also positive for MT in a Crohn's disease case. We conclude that the presence or absence of MT in lymphocytes, detected by immunohistochemistry, may indicate the growth patterns of these cells. The distinct pattern of MT staining in malignant lymphoma in our study is suggestive of a potential use of MT staining as a discriminator between benign and malignant lymphoid tissues.