Three antibiotics (avoparcin, flavomycin and wirginia- mycin) used as fodder additives for poultry were compared. The antibiotics were injected into the yolk sac of six-day-old chick embryos. Gross lesions in dead embryos, hatching, total protein content, activity of A1AT, AspAT, AP, BUN and uric acid were examined in chickens. Wirginiamycin appeared to be the most toxic for chick embryos. Avoparcin and flavomycin damaged lever and kidneys in dead embryos and affected down in hatched chickens. Avoparcin decreased hatching by 16.7%. A decreased serum activity of AspAT, BUN (p<0.05) and uric acid (p<0.01) may reflect strong disturbances of cell metabolism. Flavomycin disturbed the hatching schedule and lowered hatching by 24.71%. The increased A1AT (p<0.05) in chicken sera reflects liver damage.