Spleen cells from tumor-bearing mice showed decreased natural killer (NK) activity and decreased binding to target cells with progression of the tumor. Treatment of spleen cells from tumor-bearing mice with vibrio cholerae neuraminidase (VCN) increased the cytotoxicity to a level twice or more as high as that of untreated cells, but the same treatment of spleen cells from normal mice had no or little effect. On the other hand, neither in spleen cells from tumor-bearing mice nor in those from normal mice, the VCN treatment had no effect on their binding to M-HeLa cells. The suppression of NK activity by preincubation with serum from tumor-bearing mice or prostaglandin E2 was completely abolished by VCN treatment. The above results indicate that VCN treatment of lymphocytes might augment NK activity by an antagonistic effect against an immune suppressive factor.</p
