Abstract: The differential diagnosis between malignant and benign diseases with pleural or abdominal fluid is A common clinical problem. 73 specimens of pleural （64 cases） or abdominal （9 cases） fluid hare been examined for FN by immunodiffusion method. In 33 cases of malignant disease, the FN in pleural fluid was 648.18 ± 188.83 ug/ml （mean±SD）, in 31 cases of tuberculosis, 432.45± 109.91 ug/ml, and in 9 cases of cirrhosis the FN in asites was 191.78± 22.26 ug/ml. The results suggest that increased FN concentration reflects the presence of malignant disease, and in liver disease, particularly in cirrhosis, the FN concentration is lowered. So, it is believed that assay of FN in pleural and abdominal fluid may be useful in the differential diagnosis of diseases such as neoplasm.