An immunohistochemical technique using the unlabeled antibody enzyme method was applied to tissue sections for identification of cells according to their surface antigens. The method and its diagnostic possibilities were demonstrated on four types of malignant lymphomas. Follicular centroblastic-centrocytic lymphomas contain T-cell regions distinctly separated from the follicular areas containing B-lymphocytes staining for one immunoglobulin class only. Lymphoplasmacytoid lymphomas have a small number of T-lymphocytes distributed between tumour cells as single cells or as small clusters. Whereas the majority of tumour cells showed ringlike labeling with anti-immunoglobulin, a small number of cells revealed diffuse cytoplasmic Ig-staining. Two cases with tumorous infiltrations of non-lymphatic tissues could be ascribed to ALL because they contained tumour cells stained with AcALLG. In one of them perivascular tumour cells positive with both AcALLG and anti-IgM were classified as pre-B cells.