The in situ identification of lymphocyte subpopulations by means of immunopathological techniques using specific monoclonal antibodies provides a tool for the study of the gastrointestinal-associated lymphoid tissue (GALT) in health and disease. In this field, monoclonal antibodies have been applied previously using light microscopy and either immunofluorescence or immunoperoxidase; however, these techniques are not sensitive enough to allow precise evaluation of localization of labelling. We describe an immunoelectronmicroscopic method, which defines labelling specificity, since it allows the identification of cells by immunophenotype labelling and ultrastructural markers simultaneously. This in turn allows a better evaluation of the labelled cells and of the relationship between labelled and unlabelled cells. The main features of the method are the use of fresh tissue samples, fixing in paraformaldehyde CaCl2, and the coupling of the immune reaction to an amplification system (avidin-biotin-peroxidase complex). The technique yields a good preservation of cellular ultrastructure, together with a strong and specific immunolabelling. Our results confirm the high specificity of monoclonal antibodies when applied to immunopathology techniques. We confirm the pattern of distribution of various lymphocyte subsets in the jejunal mucosa described by other authors by light microscopy.