The object of this study were tanned hides of low black and white race cattle. During the investigations, there were determined changes in the composition of proteins contained in bovine hides that occurred during the individual phases of beamhouse processes. The following parameters were determined in the hides: dry matter, total protein, collagen, total amount of extractable protein, free fat, mineral components, and, also, the shrinkage temperature was taken. It was found that the beamhouse processes caused high quantitative changes in bovine proteins. The highest protein loss was noticed during the liming and bating. It was also observed an explicit impact of liming substances on the composition of a bovine hide: along with the increase in a Na₂S concentration, there was an increase in the amount of collagen passing into the solution. A concentration of 0,12% of Na₂S in the bating bath was the critical value; if this value was exceeded, a drastic degradation of protein occurred. The bovine collagen was more resistant to the action of Ca(OH)₂ than of Na₂S. The temperature of liming significantly influenced the content of protein extractable during the bath, thus, it should not exceed 30ºC. A close dependence between the shrinkage temperature and the quantity of collagen was observed: with the decrease in the content of collagen in hide, the shrinkage temperature sank during the individual phases of beamhouse processes. The conclusion is that beamhouse processes during which hides are prepared for tanning cause a percentage increase, by approximately 14%, in the collagen content in general protein, and this fact explains the increase in reactivity of liming leather to mineral tannins.