The work comments on the results of four experiments on biological responses of growing turkeys to fresh or oxidized fat applied in a concentrate mixture in amounts from 2% at the beginning (1–4 weeks) to 5% in the end period of fattening (13–16 weeks). The content of fat with a high peroxide value (50–150 meq O2/kg), which contained simultaneously a higher content of selenium (0.3 mg/kg), vitamin E (30–40 mg/kg) and vitamin A (1200–15000 IU/kg) caused reduction in body weight gain of turkeys, mainly as a result of lowered feed intake. Enriching diets with fat of a very high degree of oxidation (100–150 meq O2/kg) did not result in worsening of the turkeys' health condition, but considerably lowered their vitamin E reserves. Symptoms of muscular dystrophy and changes in biochemical and enzymatic indices in serum and liver were not observed. However, a decrease in the activity of lactic dehydrogenase in liver lesion of hepatocytes was noted, that was partially confirmed in microscopic assay and ultrastructural characteristics of liver. The rise in temperature (from 23°C to 30°C) in the rearing pens reduced the effect of the turkey feeding, while not, at the same time increasing a negative effect of oxidized fat on the fattening results. Products of fat oxidation increased the birds' susceptibility to virus infections causing haemorrhagic intestinal inflammation.