The effect of feed mixture supplementation with MgCl 2 on alimentary tract microflora, haematological markers, and selected parameters of reproductive performance of sows was investigated. The animals were divided into control group (K) and two experimental groups (E 1 and E 2 ). The sows from the experimental groups were receiving 1 g of MgCl 2 6H 2 0/100 kg b.w./day, which constituted 120 mg of pure magnesium, administered in a small portion of a feed mixture. The sows from group E 1 were receiving MgCl 2 every day for 30 d, whereas sows from group E 2 for 60 d. A significant decrease in the count of E. coli, Providencia sp., and Proteus sp. was noted in faeces of the swine of both experimental groups. The study has also demonstrated a reduced number of lymphocytes and an increased number of granulocytes in blood of the sows receiving MgCl 2 . Feeding of pregnant sows with the addition of magnesium salt decreased the mortality rate of newborn piglets, increased the survival rate of piglets until 21 d of their life, and increased body weight values of litters originating from experimental sows, compared to the litters of control sows.