Mounting evidence suggests that natural killer cells (NK) play an important part in the destruction of incipient tumours. If that is so, the destruction of NK cells in vivo should allow tumour cells to proliferate. We have recently described an antiserum having anti-NK activity which was raised against mouse brain tissue, and its effective antibody was shown to be directed against the cell surface glycolipid GM1 with its sialic acid groups removed (asialo GM1)¹. We show here that intravenous (i.v.) injection of microlitre amounts of anti-asialo GM1 into BALB/c nu/nu mice almost completely abolishes NK activity and enhances the local growth of syngeneic lymphoma, RL♂1 as well as the incidence of tumour take. Evidence suggests that this effect is due to membrane damage producing loss of NK function rather than to cytolytic membrane attack.