Abstract： Objective To establish a method (negative enrichment by immunomagnetic beads) for detection of tumor cells in pleural effusions and to evaluate the sensitivity and specificity of the method for clinical application. Methods Five,10,20,50 and 100 A549 (lung adenocarcinoma) cells were labeled with DAPI and added into 20 ml pleural effusions [ containing ( 1 - 10 ) × 106cells ]from heart failure patients,followed by immunomagnetic negative enrichment method. Recovered cancer cells were enumerated using a fluorescent microscope.Tumor cells were enriched from pleural effusion samples by means of density gradient centrifugation and negative enrichment by immunomagnetic beads method,followed by identification with cytology analysis (Wright' s Giemsa' s staining),immunofluorence staining (IF) and fluorescence in situ hybridization (FISH) using centromere DNA probes of chromosome 7 and 8. Cytology,IF and FISH evaluations were performed in 53 pleural effusion samples,including 36 cases of malignant disease (25 male and 11 female patients aging 40 to 78 years,mean age (63 ± 9) and 17 cases of benign disease (8 male and 9 female patients aging 25 to 81 years,mean age (53 ± 18).Results After DAPI staining and mixing with pleural effusions from heart failure patients, the cell recovery rates of A549 cells evaluated under fluorescence microscope were 75％,78％,82％,85％,88％,and the average recovery rate was 81.6％.Using negative enrichment method and density gradient centrifugation combined with cytology analysis,the positive rates of tumor cells in 36 malignant pleural effusion samples were 81％ (29/36) and 61％(22/36),respectively (x2 =4.00,P =0.039). Using negative enrichment method combined with IF,the positive rate of CK18( + ),DAPI( + ),CD45 ( - ) cells was 100％.Moreover,using negative enrichment method combined with FISH analysis,the positive rate of tumor cells was 86％ (31/36),much higher than that using density gradient centrifugation combined with cytology analysis ( x2 =5.818,P =0.012 ). In 17cases of benign pleural effusions,using negative enrichment method combined with IF,the positive rate was 100％. But other methods didn' t find cancer cells from benign pleural effusions. Conclusions It was applicable to enrich tumor cells from pleural effusions using negative enrichment method by immunomagnetic beads. This method combined with cytology analysis or FISH significantly enhanced the sensitivity and specificity of tumor cell detection in pleural effusions. But it was difficult to distinguish cancer cells from mesothelial cells using immunofluorence staining with CK18,DAPI and CD45 label. More specific markers were needed to recognize tumor cells from pleural effusions.