Natural Killer (NK) cell activity of 47 operable stomach cancer patients was sequentially studied before and after chemotherapy in association with polyadenylic. polyuridylic acid [poly(A).poly(U)]. NK activity was determined by an in vitro 4 h chromium release assay using nonadherent mononuclear cells isolated from peripheral bloods as effectors and human myeloblastic cells (K562) as targets. The following results were obtained: 1) The mean NK activity of the 47 patients tested before chemotherapy was significantly lower than that of 14 healthy controls. 2) The patents who received chemotherapy consisting of 5 FU (12 mg/kg) and adriamycin (40 mg/M²) showed an increase in NK activity 5 days after injection as compared to that of the same patients tested before chemotherapy. 3) In these patients, an additional administration of poly(A) poly(U) (100mg) resulted in a further significant increase of NK activity 2 days later, whereas the control patients who received placebo showed no change of NK activity.