Nine healthy middle-aged and 10 elderly volunteers drank fermented milk containing 4 x 10(10) live cells of Lactobacillus casei strain Shirota daily for 3 wk, and their natural killer (NK) activity and other immunological functions were examined. In the experiments with middle-aged volunteers, NK activity significantly increased (P<0.01) 3 wk after the start of intake, elevated NK cell activity remained for the next 3 wk, and this effect was particularly prominent in the low-NK-activity individuals. In the experiments with elderly volunteers, NK activity significantly decreased (P<0.01) in the control group 3 wk after the start of intake; however, the intake of Lactobacillus casei strain Shirota maintained the NK activity. These results suggest that daily intake of Lactobacillus casei strain Shirota provides a positive effect on NK-cell activity.