We investigated the effect of recombinant interferon-gamma (rIFN-gamma) in vitro on natural killer (NK) cell activity of peripheral blood lymphocytes (PBL) in patients with gastric cancer. Thirty untreated patients with gastric cancer and eleven healthy donors were studied. NK cell activity in healthy donors was 38.6 +/- 5.2%. In patients with gastric cancer, NK cell activity was 32.4 +/- 3.9% in stage I, 35.5 +/- 7.1% in stages II and III, and 22.4 +/- 3.9% in stage IV. NK cell activity in stage IV patients was significantly lower than in healthy controls or gastric cancer patients in stages I-III. After stimulation by rIFN-gamma, NK cell activity increased in all groups, and in stage IV patients NK cell activity recovered to normal levels. However, there was no significant increase in the proportion of Leu 7 positive or Leu 11 positive cells in PBL. In conclusion rIFN-gamma enhances the activity of NK cells, but does not increase the number of active NK cells.