Abstract: The aim of this study was to examine the effect of molybdenum (Mo) on selected haematological parameters of porcine blood in vitro. The samples of blood were treated with an ammonium molybdate (NH4)6.Mo7O24.4H2O) for 4 hours at 37oC in the concentrations 10, 100, and 1000 µg.ml-1 (E1, E2, and E3 group). Blood without Mo addition served as the control (C). Selected haematological parameters (WBC – total white blood cell count, LYM – lymphocyte count, GRA – granulocyte count, RBC – red blood cell count, HGB – haemoglobin, HCT – haematocrit and PLT – platelet count) were measured using haematological analyser Abacus junior VET. Significant decrease in WBC, LYM, and GRA in the group with the highest dose of Mo (E3) against other groups (C, E1, and E2) was observed. Other results were not influenced by Mo exposure. Our results suggest that Mo can cause the changes and imbalance in immune system.