1. The aim was to evaluate the functional efficiency of a probiotic strain Enterococcus faecium M 74 in the feed on selected biochemical, haematological and production parameters of ISA Brown hens. 2. Feed in the experimental group was enriched with a probiotic preparation containing of 5 × 10(9) viable E. faecium M 74 per g. Blood samples were collected during the egg-laying period at 5 (w5), 25 (w25) and 45 (w45) weeks of production. Body weight, rate of lay and egg weight were recorded every 4 weeks during the 48-week laying period. 3. Significantly lower concentrations of total cholesterol and total lipids in blood plasma were observed in the experimental group at all sampling times compared with their respective controls. Concentrations of triglycerides did not differ. Significantly lower concentrations of plasma calcium were found in the experimental group at w5 and w45. Concentrations of inorganic phosphorus in the experimental group were significantly higher at w25, but significantly lower at w45. Erythrocyte count was significantly higher in the experimental group at w25 and w45 when compared with controls. Leucocyte counts were significantly lower in the experimental group at all sampling times. Significantly lower values of haematocrit at w5 and w45 were observed in the experimental group than in controls. Body weight, the number of eggs and average egg weight were not significantly affected by probiotic addition. 4. In conclusion, the addition of probiotic strain E. faecium M 74 to the feed of ISA Brown hens reduced cholesterol, lipids, calcium, leucocyte counts and haematocrit values in blood plasma in at least two sampling times, while erythrocyte counts were increased. No significant effects of probiotic on triglyceride concentration and egg production parameters were observed.