1. This experiment was to investigate the effects of natural dietary contamination with a mycotoxin product (deoxynivalenol: DON) and/or with dietary selenised yeast (Se-yeast), on respiratory burst and phagocytic activity of granulocytes and the frequency of B- and T-lymphocytes in peripheral blood of broilers. 2. Sixty one-day-old chicks of both sexes were divided into 4 groups, each of 15 birds, fed on a control diet that contained 0.2 mg DON/kg and 0.4 mg Se/kg (CON group), a diet supplemented with 1 mg Se-yeast/kg (Se-yeast group), a diet contaminated with 3 mg DON/kg (DON group) or a diet contaminated with DON and supplemented with Se-yeast (DON plus Se-yeast group). 3. Blood samples collected from the birds at the age of 4 weeks showed that neither B- and T-cell numbers nor granulocytic respiratory burst were influenced by 3 mg DON/kg. Blood granulocyte phagocytic activity was not reduced by DON but numbers of heterophils were increased. In the DON plus Se yeast group phagocytic activity was the same as in the CON group. The Se-yeast and DON plus Se-yeast groups had increased numbers of CD3(+), CD4(+), and CD8(+) T-cells as well as IgM(+) B-cells in their blood compared to both CON and DON-groups. 4. The results show there is no significant effect of dietary DON up to 3 mg/kg on leukocytes apart from the compromised blood granulocytes phagocytic activity and increased numbers of heterophils. The increased numbers of B- and T-lymphocytes in blood of birds fed on diets with supplementation of organic Se indicates some positive effects of this essential microelement on poultry lymphoid cells.