Distinguishing reactive mesothelial cells from adenocarcinoma cells in serous effusions on the basis of morphological criteria alone is often difficult. Interest has therefore been focused on identifying reliable methods to supplement the conventional cytological techniques. Plant lectins have been reported as diagnostic markers for malignant cells. We studied 51 aspirated samples of benign and malignant effusions using horseradish-peroxidase-conjugated jackfruit lectin. No significant difference was observed between the cells of pleural and peritoneal fluids. The reactively proliferated mesothelial cells of benign effusions showed a predominance of mild staining while moderate and intense staining was predominant in malignant effusions. Intense and irregular lectin binding was observed in macrophages irrespective of the cause of effusion. The lectin staining method therefore appears to have some clinical significance as an additional diagnostic aid for use in effusion cytology.