We investigated the role of graft luminal fluid cytology for immunological monitoring of rat small-bowel allograft recipients. Allogeneic transplantation from WKAM (RT1u) to Lewis recipients and syngeneic transplantation using Lewis (RT11) rats were carried out. Twenty centimeters of the proximal jejunum was transplanted as a Thiry-Vella loop. The luminal fluid on days 0, 3, and 6 was examined cytologically using Papanicolaou, periodic acid-Schiff, and Giemsa staining, and immunocytochemically with monoclonal antibodies for macrophages (ED1 and ED2). Full thickness biopsies of graft tissue were evaluated by both immunofluorescence (ED1 and ED2) and by standard histological methods. The cytological examination on day 6 revealed an increase in the number of enterocytes, lymphocytes, and neutrophils, the presence of bacteria, and the depletion of goblet cells in the allografts. Histologically, significant morphological changes of acute rejection were first seen on day 6. Immunofluorescence predicted the acute rejection of the allografts earlier than a histological examination by showing an increase in the number of ED1- and Ed2-positive cells on day 3. Graft luminal fluid cytology and immunofluorescence analysis of ED1 and ED2 cells could thus be used to recognize early acute allograft rejection following small-bowel transplantation.