Introduction: The transition period between late pregnancy and early lactation (also called the periparturient period) certainly is the most interesting stage of the lactation cycle and is the last 3 weeks before parturition to 3 weeks after parturition. Most infectious diseases and metabolic disorders occur during this period. Milk fever, ketosis, retained fetal membranes, metritis, and displaced abomasum primarily impact cows during the periparturient period (Drackley 1999). Any nutritional limitation during this period has an important impact on cow efficiency and consequently, milk production decreases. Dairy cows encounter substantial metabolic and physiological adaptations during the transition period. The immune system during the periparturient period is impaired. At this time, the most important factor causing immune-suppression in highly productive cows is metabolic stress resulting from hormonal and metabolic fluctuations, a negative energy balance, shortage of proteins, minerals and vitamins which are required to meet the demands of the fetus as well as the onset of lactation (Sordillo 2016). In the world, Cu deficiency is one of the most common problems in cattle with clinical and subclinical signs (Hill and Shannon 2019). Even marginal Cu deficiency (6 to 7 ppm dietary Cu) depresses blood neutrophil function in dairy cattle (Torre et al. 1996). One strategy for improving immune system of transition dairy cows is mineral and vitamin supplementation. It has been reported that many minerals are enzymatic cofactors (Filappi et al. 2005). Studies showed that minerals injection would be a suitable method to improve mineral utilization, and this may be a promising alternative to improve animal performance (Collet et al. 2017). Furthermore, during this period, dairy cows need antioxidants to combat reactive oxygen species (ROS) which produce during oxidative stress. Vitamin C was identified as antioxidant and could help immune system to overcome ROS production. Hence, this experiment was aimed to study the effect of vitamin C and copper injection on the health status of transition dairy cows. Materials and Methods: The study was carried out in a commercial farm located in Kermanshah province of Iran. Cows were enrolled from June 22, until September 22. Temperature and humidity index (THI) was calculated. It was between 72-78. The experiment was performed with 40 multiparous (twenty; second parity and twenty; third and fourth parity) Holstein lactating dairy cows which divided into four groups (10 animals/ group) in a 2×2 factorial arrangement. All cows were offered a TMR diet. Experimental treatments consisted of control (injection of 7 ml of NaCl % 0.9), Cu (injection of 75 mg Cu per cow) Vitamin C (injection of 25 mg vitamin C solution/kg BW), and Vitamin C-Cu (simultaneous injection of 25 mg vitamin C solution/kg BW and 75 mg Cu/ cow). Solutions were injected on d 20 and 40 days before expected parturition, parturition day and day 20 of postpartum. Serum concentrations of total protein, glucose, triglycerides, cholesterol, HDLcholesterol, Ca, P and Mg were determined using autoanalyzer by Pars Azmoon Kits. Serum concentrations of Beta hydroxybutyrate (BHBA) and non-esterified fatty acids (NEFA) were measured using autoanalyzer by Randow Kits. Serum concentration of Cu was determined by atomic absorbtion. Serum concentrations of vitamin C and superoxide dismutase was measured by Elisa reader. Incidence of metabolic disorders and infection disease were recorded. Data of serum variables were analyzed based on a randomized block design with a 2×2 (Vitamin C and Cu, with or without injection) factorial arrangement using Proc Mix of SAS software. The differences among treatments were evaluated using Tukey adjustment when the overall F-test was P ≤ 0.05. Trends were declared when 0.05 < P < 0.10. In addition, percentages of metabolic disorders were reported. Results and Discussion: Results showed that the interaction effect of vitamin C and Cu had no significant effect on serum concentration of vitamin C and superoxide dismutase activity. Cows received vitamin C had the greatest serum vitamin C concentration on d 20 prepartum, parturition day, d 10, 20 and 30 postpartum (P<0.05). Content of serum Cu were affected by the interaction of vitamin C and Cu on d 10 and d 20 postpartum (P<0.05). Copper injection increased serum Cu concentration and superoxide dismutase activity on d 20 prepartum, parturition day, d 10, 20 and 30 postpartum (P<0.05) in cows received Cu without vitamin C. Serum concentrations of P, Mg, glucose, BHBA, NEFA, triglycerides and HDL-cholesterol were not influenced by the interaction effect of vitamin C and Cu. Serum triglyceride concentration tended to decrease (P=0.06) in cows received vitamin C and on parturition day. Copper injection tended to increase (P=0.06) serum triglyceride concentration on d 20 postpartum. Serum protein concentration tended to decrease (P=0.07) in cows received vitamin C and in compared to other treatments on parturition day. Copper or vitamin C injections had no effect on serum concentrations of Ca, P, Mg, glucose, BHB and NEFA of experimental cows. Serum calcium concentration tended to increase (P=0.07) in cows received simultaneous injection of vitamin C and Cu in compared to the others on parturition day. Simultaneous injection of vitamin C and Cu tended to increase total cholesterol concentration (P=0.06) on d 10 and 30 of postpartum. Copper injection tended to increase total cholesterol concentration on d 20 prepartum, parturition day and d 10 postpartum (P=0.06). Control groups had the highest incidence rate of retained placenta and clinical mastitis. Conclusion: It is concluded that vitamin C and Cu injection reduced incidence rate of retained placenta and clinical mastitis due to improving immunity system performance of transition dairy cows. [ABSTRACT FROM AUTHOR]