The cytological distinction of carcinoma cells from reactive mesothelial cells in serous effusions nay be difficult or imposslble based on morphology alone, especially In specimens containing reactive mesothelial cells which form glandular or ball- or papillary-shaped conglomerates or which mimic malignant nuclear features. Calretinin is a newly reported immunocytochemical marker for mesothelial cells, which can potentially be utilized for facilitating this distinction. This study evaluated the usefulness of calretinin for the discrimination between reactive mesothelial and metastatic carcinoma cells in serous effusion. Immunocytochemical staining was undertaken on 33 benign reactive and 87 malignant serous effusion specimens with histologically confirmed diagnoses. The specimens including smears and cell blocks were stained with polyclonal antibody to calretinin by labelled streptavidin-biotin method. The positive expression of calretinin was noted In 32(97.0%) of 33 benign reactive effusions and 9(10.3%) of 87 malignant effusions. The sensitivity and specificity of the calretinin immunostaining for reactive mesothelial cells was 97.0% and 89.7%, respectively. In conclusion, calretinin is a useful marker for distinguishing between reactive mesothelial cells and carcinoma cells in serous effusions.