The natural killer (NK) activity of blood mononuclear cells and splenocytes in aged mice fed on a diet containing Lactobacillus casei strain Shirota (LcS group) was significantly (P < 0.01) higher than that in mice fed on a diet without LcS. In the LcS group, there was a significant positive correlation (r = 0.63, P < 0.01) between the NK activity of blood mononuclear cells and the NK activity of splenocytes.