The immune fuction of 41 patients with duct cell adenocarcinoma of the pancreas or carcinoma of the periampullary region was studied by skin testing with 2,4-dinitrocholorobenzene and common microbial antigens, in vitro lymphocyte reactivity to mitogens, T-lymphocyte and B-lymphocyte cell counts and measurements of complement levels. Results show that a great proportion of patients had depressed immune function. Depression is particularly severe in lymphocyte reactivity. Positive response to skin tests with a microbial antigen carries a slightly better prognosis. The complement level is usually normal or elevated. Correlation between immune function, prognosis and tumor resectability in individual patients is generally poor.