The effects of orthotopic liver transplantation (OLTx) and cytomegalovirus (CMV) gastroenteritis on the type of mononuclear cells within the upper gastrointestinal tract were determined. Nineteen liver transplant recipients were studied both before and after transplantation. Each underwent a pan-upper gastrointestinal endoscopy with biopsy of the antrum and duodenum before and four weeks following liver transplantation. A panel of monoclonal antibodies prepared against HLA-DR, NK, IL-2R, T11, T4, T8, and B1 cell surface antigens was used to examine the tissues. Before OLTx, none of the 19 subjects studied had clinical or histologic evidence for CMV gastroenteritis. Following OLTx, five of the 19 subjects had CMV gastroenteritis. The number of HLA-DR positive staining lymphocytes present in biopsies obtained post-OLTx was significantly greater (P less than 0.005) than those present in biopsies obtained pre-OLTx regardless of the presence or absence of CMV gastroenteritis. No difference in the intensity of HLA-DR antigen expression between pre- and post-OLTx biopsies and those with and without CMV gastroenteritis was evident. No difference in the number of natural killer (NK) cells and the number of cells expressing the interleukin-2 receptor (IL-2R) was evident between biopsies obtained pre- and post-OLTx. In contrast, the number of T lymphocytes bearing the T11, T4, and T8 markers and the calculated T4/T8 ratio differed between biopsies obtained pre- and post-OLTx and between those positive for CMV gastroenteritis post-OLTx and those without evidence for CMV gastroenteritis either before or after OLTx, although these changes were not consistent throughout the gastrointestinal tract.(ABSTRACT TRUNCATED AT 250 WORDS)