The paper presents results of research on the effects of different doses of copper in the form of CuSO4·5H2O and Cu chelate and lysine (1:2), also with 0.4% lactic acid in drinking water on weight gain of turkeys. The study sample consists of 20 experimental groups of 120 birds each. The addition of lactic acid to the feed (excluding copper) contributed slightly to the reduction of the weight gain of turkeys. In contrast, a significant decrease in body weight gain of turkeys was observed, using lactic acid along with inorganic form of copper. Turkeys fed with the feed containing only organic form of copper at a dose of 20 mg reached the greatest weight gain. Lactic acid and different doses and forms of copper significantly differentiate the body weight gain of turkeys. The combination of the lactic acid and copper added resulted in lower weight gains of birds; it was only the administration of the 50 mg copper dose in organic form that increased the weight gain to the level comparable to the weight gains obtained for birds fed without the lactic acid addition. The use of feed supplements requires the selection of an appropriate dose to the form of copper.