To explore im mune intervention effects of the combined use of cycloporin A (Cs A ) and 1,2 5 - dihydroxyvitam in D3[1,2 5 (OH ) 2 D3]at low doses on experimental autoim mune thyroiditis (EAT) ,porcine thyroglobulin (p TG) was injected into a CBA mouse at the dose of 10 0μg on day 0 and day14 to establish the model of EAT.The immune prevention group from day0 to day2 8, and treatm ent group from day10 to day38were daily adm inistered Cs A(10 mg/ kg) intragastri- cally and/ or1,2 5 (OH) 2 D3(0 .2 μg/ kg) ip.After im munized by p TG,the m ice were sacrificed on day2 8and day38to examine their thyroid gland pathologically,and to check the levels of serum porcine thyroglobulin antibodies (p TGAb) ,porcine thyromicrosom al antibodies (p TMAb) .The incidences of EAT in the immune prevention group and treatment group,with administration of low dose of Cs A and 1,2 5 (OH) 2 D3,were decreased respectively by 4 4 .4 4 % and 37.5 0 % .Those of severe disease in the two groups were decreased respectively by71.4 3% and6 0 .32 % .The levels of serum p TGAb and p TMAb in the imm une prevention group were lower than those of the positive control group.It was concluded thatcombined use of Cs A and1,2 5 (OH) 2 D3at low doses could effectively prevent EAT with a synergic effect. To explore im mune intervention effects of the combined use of cycloporin A (Cs A) and 1,2 5-dihydroxyvitamin in D3 [1,2 5 (OH) 2 D3] at low doses on experimental autoim mune thyroiditis (EAT), porcine thyroglobulin (p TG) was injected into a CBA mouse at the dose of 10 μg on day 0 and day 14 to establish the model of EAT. The immune prevention group from day0 to day2 8, and treatm ent group from day10 to day38were daily adm inistered CsA (10 mg / kg) intragastri-cally and / or 1,25 (OH) 2 D3 (0.2 μg / kg) ip. After im munized by p TG, the m ice were sacrificed on day2 8 and day38 to examine their thyroid gland pathologically, and to check the levels of serum porcine thyroglobulin antibodies (p TGAb), porcine thyromicrosom al antibodies (p TMAb). The incidences of EAT in the immune prevention group and treatment group, with administration of low dose of Cs A and 1,2 5 (OH) 2 D3, were expressed respectively by 4 4 .4 4% and 37.5 0%. Those of severe disease in the two groups were dec reased respectively by 71.4 3% and 6 0 .32%. The levels of serum p TGAb and p TMAb in the imm une prevention group were lower than those of the positive control group. It was concluded that combined use of Cs A and 1,25 (OH) 2 D3at low dose could effectively prevent EAT with a synergic effect.