Thirty two 3 months old Polish Lowland wethers of about 20 kg initial body weight were divided into four equal groups. Wethers in each group were fed individually for 78 days with diets consisting of dried sugar beet pulp and meadow hay supplemented with untreated or formaldehyde-treated casein and untreated or formaldehyde-treated rapaseed oil meal. All diets contained about 20% crude fibre and 17% crude protein (DM basis), about 50% of that protein was derived from casein or rapeseed oil meal. Dry mater and protein digestibility of diets containing formaldehyde treated feeds was less (P < 0.05) and animal body weight gains were lower (P < 0.01) than those with untreated feeds. Body gains were 20% higher in groups fed rapeseed oil meal comparing to those fed diets with casein (P < 0.01). Differences in length, weight and thickness of wool were not significant (P > 0.05) between animals receiving untreated or formaldehyde-treated protein.