The influence of formic acid added in three doses (3,5 and 7 g/kg fresh mass) in the intensity of proteolysis and fermentation in lucerne silages was investigated. On the basis of chemical analyses, it is observed that with the increase of conserving dose the lower pH value, limited proteolysis and increased preservation of proteins was achieved. The amino acid content varied very much due to their degradation and biosynthesis. In treated silages a statistically significant increase of lactic acid was observed while silages with maximum dose of conserving had significantly less free and total acetic acid. Presence of bonded butyric acid only in the control silage had no statistical significance. According to Dulphy and Demarquilly (1981) method, control silage with minimum dose of conserving was estimated as III class and silages with medium and high dose of formic acid were estimated as II quality class.