Folate, a precursor of the coenzyme tetrahydrofolate, plays an important role in DNA replication and cell proliferation, and thus could influence rapidly proliferating immune cells such as leukocytes and splenocytes. The effects of dietary folate on folate concentrations of plasma, thymus, spleen and leukocytes were investigated in rats. The animals were raised for 6 weeks on semipurifed experimental diets containing 0mg, 2mg, 4mg, 8mg folate/kg diet. Folate concentrations were determined microbiologically using Lactobacillus casei(ATCC 7469), and DNA strand breaks produced by alkaline treatment were analyzed fluorometrically. When compared to folate adequate diet, the folate deficient diet(0mg folate/kg diet) resulted in lowest folate levels in plasma, thymus, spleen and leukocytes, and the highest DNA strand breaks in spleen cells and leukocytes. Dietary folate levels significantly increased folate concentrations of immune tissues, leukocytes, and the plasma in a dose dependent manner, folate concentrations being highest with a diet providing 8mg folate/kg diet. The percentages of the double strand DNA remaining in the splenocytes and leukocytes after alkaline treatment were significantly increased with higher amounts of dietary folate in a dose dependent manner. Folate intakes of 8mg than 4mg/kg diet was found to be more effective in the prevention of DNA strand breaks. The results of this study suggest that increased folate above the requirement level could improve DNA stabilities in immune cells.