Aims and Background The usefulness of monoclonal antibodies that recognize markers of neoplastic lesions in complementing conventional cytology was evaluated by the avidin-biotin-peroxidase complex, indirect immunoperoxidase technique. Methods In order to enhance the sensitivity of the traditional method, a pool of seven combined monoclonal antibodies (Pool C7), which reacts specifically with cells of epithelial origin and is able to distinguish between mesothelial and malignant cells, was tested on cytologic smears of 262 serous effusions. The effusions were benign or neoplastic, mainly from breast, ovary and lung cancers. Results Immunocytochemical method showed an 100 % specificity and 100 % of predictivity whereas the sensitivity was 98 %, 96 % and 95 % for breast ovarian and lung carcinomas, respectively. Conclusions The results demonstrated that the pool when used together with conventional methods, is useful in analysis of serous effusions in diagnostic investigations.