An in vivo microscopic technique is described that was developed to measure the interaction between transferred donor lymphocytes and islet endothelium in diabetes transfer experiments. With double fluorescent tracer staining by acridine red and FITC-dextran the simultaneous assessment of lymphocyte adhesion and shear forces acting upon adherent cells was possible. Staining the isolated donor lymphocytes with acridine red did not significantly affect their viability. Proliferation capability was maintained after the staining procedure. By using exocrine pancreatic tissue as a reference vascular bed, the specificity of the observed changes in lymphocyte adherence in pancreatic islets could be demonstrated. This new approach in microcirculatory research will help to clarify the mechanisms underlying the initial steps of lymphocyte infiltration into islet tissue in autoimmune diabetes mellitus.