To elucidate health benefits of beta-carotene, especially on immunity, we measured redox-related indices in spleen cells from BALB/c mice supplemented with various amounts of beta-carotene. In mice supplemented with beta-carotene in their diet, glutathione, an intracellular anti-oxidation agent, increased in their splenocytes. This change was highly correlated with the accumulation of beta-carotene, but not with that of retinol. The increase in glutathione was accompanied by an increase in mRNA for gamma-glutamylcysteine synthetase, a rate-limiting enzyme for glutathione synthesis. The higher the glutathione content was in the spleen cells, the higher the activity of cysteine cathepsin became in crude antigen-presenting cells contained in the spleen. These data suggest that accumulated beta-carotene in splenocytes, without being metabolized, caused an increase in the intracellular glutathione level, thereby anti-oxidatively supporting the activity of redox-sensitive lysosomal protease, which is involved in antigen-presentation.