The immunocytochemical characterization of neoplastic cells recognized by a large panel of monoclonal antibodies in fine needle aspirates obtained from 16 subjects with superficial or profound tumoral masses of unknown origin was carried out using alkaline phosphatase anti-alkaline phosphatase (APAAP) immunoenzymatic method. The patients were selected on the basis of particular clinical criteria or since a correct cytohistological diagnosis was not available. APAAP immunophenotyping allowed to establish the existence of non-Hodgkin lymphoma in 6 cases, hairy cell leukemia in 1 case and carcinoma in 5 cases, while in other 2 patients the existence of Hodgkin's disease was suspected and subsequently confirmed; only in 2 subjects diagnostic informations were lacking. Furthermore, the evaluation of growth fraction by means of Ki 67 monoclonal antibody revealed a high degree of malignancy in all cases of lymphoma. Although some cautions have to be considered in the interpretation of results, APAAP immunocytochemical typing of fine needle aspirates appears a simple and reliable noninvasive diagnostic tool in selected patients.