Two experiments were carried out with 192 hybrid Tetra SL hens kept in cages (A) and 5340 medium-weight Hisex Brown layers kept on litter (B). Experimental diets contained 16 (A) or 18% (B) crude protein and about 11.2 MJ ME/kg. Five or 10 (A) and 10 or 20 (B) ppm of Avoparcin (Avotan) were added to the experimental diets. The laying rate, egg mass, feed intake, mortality, egg quality and Ca, inorganic-P and Mg levels in blood serum were investigated. In experiment A Avoparcin in both 5 and 10 ppm concentration did not affect the laying rate which averaged 85.7%. In experiment B the dose of 10 ppm Avoparcin increased the laying rate by 18%, 20 ppm by 6%. The improvement of feed utilization per egg by 6% and the strength of the shell by about 10% after Avoparcin addition were observed in Experiment B only. The influence of antibiotic on the mineral components of the blood was negligible and only the level of inorganic P varied.