[Objective] This study aimed to research the effect of metallothionein on cell cycle, apoptosis rate and subsets distribution of lymphocytes in peripheral blood of dairy cows under heat stress, so as to perfect the regulative mechanism re- searches of MT to anti-heat stress. [Method] Twenty lactating Chinese Holstein cows were randomly divided into four groups （A, B, C and D）, and injected with 0, 4.0, 8.0 and 12.0 mg Zn-metallothionein, respectively by intravenous route. Blood sam- ples were collected at 1＂, 16~, 31~, 46~ and 61~ day, and the dynamic changes of cell cycle, apoptosis rate and subsets distribution of lymphocytes were determined. [Result] The apoptosis rate of cells in group B and C was lower than those in group A by 26.63% （P〉0.05） and 24.84% （P〉0.05） respectively. The number of cells in the Gc/G1 phage in trial groups was increased and the number of cells in the S and GJM phages tended to decrease, but there were no significant differences （P〉 0.05）. The number of CD3~ T cell in three trial groups was greater than those in group A by 7.02% （P〉0.05）, 5.45% （P〉0.05） and 3.85% （P〉0.05） respectively, while the number of CD4~ T cell in trial groups was higher than those in control group by 31.04% （P〈0.05）, 35.68% （P〈0.05） and 39.34% （P〈0.05） respectively. The number of CD8＇ T cell and the levels of CD4＊/CD8~ in trial groups were increased observ- ably, but significant difference （P〈0.05） was observed in the levels of CD4~/CD8~ between groups A and C only. It demonstrated that exogenous Zn-metallothionein can decrease apoptosis rate, improve cell cycle and regulate subsets distribution of lymphocytes in dairy cattle in a dose-dependent manner. [Conclusion] This study will provide scientific basis for safe utilization of MT in dairy industry.
