Objective:To explore the clinical value of qualitative determination of C-reactive protein (CRP) in the differential diagnosis of benign and malignant pleural effusions or ascites .Methods :CRP in 3820 cases of pleural effusions and ascites were qualitatively examined by latex agglutination method .Specimen and latex reagent were added to the wells of the reaction plate successively .The result was observed after 2 minutes’ thorough mixing .The result was judged as positive if agglutination par-ticles were visible .Results:The positive rate of CRP in 672 cases of malignant pleural effusions and ascites was 6 .1% .The positive rate of CRP in pleural effusions and ascites was 75 .0% in 32 cases of hematopoietic and lymphoid tissue tumors ,and 2 .7% in 640 cases of epithelial malignant tumors .The positive rate of CRP in pleural effusions and ascites in 3148 cases of be-nign diseases was 21 .8% .The positive rate of CRP in benign pleural effusions and ascites was higher than that in malignant pleural effusions and ascites(P<0 .05) .The positive rate of CRP in pleural effusions and ascites of patients with hematopoietic and lymphoid tumors was higher than that of patients with epithelial malignant tumors .Conclusions :Qualitative determination of CRP in pleural effusions and ascites can be used to differentiate benign and malignant pleural effusions and ascites ,and to differentiate pleural effusions and ascites of hematopoietic and lymphoid tissue tumors and epithelial malignant tumors .