We previously found that activated peritoneal neutrophils adhered to tumor cells and destroyed them in the cancer ascites of patients who had received intraperitoneal (ip) OK-432 injection therapy. Since tight adhesion to the tumor cell is essential for effective neutrophil-mediated tumor cell destruction, we investigated the mechanism of peritoneal neutrophil adhesion to tumor cells, using a microplate adhesion assay. An in vitro study demonstrated that the adherence activity of the peritoneal neutrophils of patients who received OK-432 injection therapy to tumor cells increased greatly compared to that of blood neutrophils. The expression of the adhesion molecules (CD11a,b,c/CD18) of peritoneal neutrophils, which was determined by an immunofluorescence study, was about four times as much in CD11b and twice as much in CD11c and CD18 compared to that in blood neutrophils. In vitro OK-432 stimulation of normal blood neutrophils increased neither the adhesion to PLC nor the CD11b expression. The enhanced adherence activity of peritoneal neutrophils to tumor cells was significantly inhibited by pretreatment of the neutrophils with anti-CD11b and anti-CD18 monoclonal antibodies (mAb), but not by pretreatment with anti CD11a or anti-CD11c mAb. These results indicated that the increased adhesiveness of OK-432-induced peritoneal neutrophils to tumor cells was due to the enhanced expression of CD11b/CD18. We concluded that CD11b/CD18 molecules on OK-432-induced peritoneal neutrophils play a crucial role in the neutrophil adherence activity against tumor cells, and these results are the first demonstration in the field of human neutrophil function.