This study investigates the applicability of immunocytochemical techniques towards improving the cytological diagnosis of lymph node disorders. Cytocentrifuge preparations of fine needle aspirates were examined using an indirect immunoperoxidase method and the alkaline phosphatase-antialkaline phosphatase method. 36 reactive lymph nodes were evaluated. 23 patients showed T cell predominance as assessed by the presence of CD3+ cells. 13 patients showed an excess of CD24+ cells over CD3+ cells. In all patients the B cell population showed no light chain class restriction. 48 aspirates from patients with B-non-Hodgkin's lymphoma of low malignancy were investigated. The majority of the cells in each of these cases expressed CD24 and 47 cases were monoclonal with respect to their light chain determinants. In 37/48 cases the antibody OKT9 reacted with less than 15% of cells. 18 aspirates were obtained from patients with B-non-Hodgkin's lymphoma of high malignancy. 17 patients showed light chain class restriction and a high percentage of CD24+ (greater than 80%) and OKT9+ (greater than 40%) cells. 8 malignancies were considered to be of T-cell origin. A low percentage (less than 15%) of Ig+CD24+ cells with a high number of CD3+ and/or CD4+ cells suggests a T-cell lymphoma. The majority of neoplastic cells in peripheral T-cell lymphomas (excluding mycosis fungoides) expressed OKT9 and CD3 expression was found to be variable. In 15 cases of Hodgkin's disease, there were 11 correct cytologic diagnoses. A significant number of BerH2 (CD30)+ cells having the morphology of Sternberg-Reed cells supports this diagnosis. 25 aspirates were obtained from patients with metastatic malignant tumours. Marker studies in eight cases helped in distinguishing metastatic malignant tumours from malignant lymphomas. Our findings indicate that the immunocytochemical technique is applicable to cytological material and that the simplicity of the procedure merits application to routine diagnostic cytology.