Macrophage surrounded by transformed or untransformed lymphocytes is called immune island. Aimming at this problem, we made a survey of 120 pleural and peritoneal effusion smears with a variety of patients and discovered that this island appeared in 52 (43.3%) cases. The positive results existed in 11 of 14 (79%) cases with tuberculosis and 29 of 60 (48%) cases with malignant diseases. The average number of island on each positive smear (2.0cm x 2.5cm) is 59.6 +/- 12.8 in the former and 39.2 +/- 18.9 in the latter. The mechanism of this phenomenon may be due to the response to foreign stimulus from pathogenic and cancer antigen.