Twelve Polish-Merino lambs, divided into two groups of 6, were fattened from 18 to 39 kg body weight on diets containing 14% crude protein and 11% crude fibre in dry matter; 25% of crude protein was from untreated lupin seeds or a mixture of lupin extruded at 160 °C with barley (1:1). Daily body weight gains of lambs fed extruded lupin seeds were higher (P < 0.5) than those of lambs led the untreated lupine-barley mixture (234 vs 186 g). Feed efficiency was 5.0 kg dry matter and 729 g crude protein in lambs fed the untreated lupine-barley mixture and 4.2 kg dry matter and 597 g crude protein in animals receiving the extruded mixture. Effective protein degradability of untreated lupin was 0.85, untreated barley 0.75 or untreated lupin-barley mixture 0.80, whereas that of extruded at 130, 140, 150 or 160 °C lupin-barley mixtures gradually decreased from 0.70 to 0.67. The respective effective protein degradability values for barley extruded at 135 or 155 °C were 0.72 and 0.60. The coefficient of intestinal protein digestion of untreated lupine was 0.59, of barley, 0.83, and of the lupin-barley mixture, 0.71, whereas for extruded lupin-barley mixtures, it only slightly increased with temperature from 0.92 to 0.94, while for extruded barley from 0.91 to 0.92. Substitution of unprocessed lupin seeds with extruded seeds decreased protein degradation in the rumen, increased protein intestinal digestibility and effectiveness of fattening lambs. Increasing the extrusion temperature to over 130 °C did not significantly improve degradability in the rumen or intestinal digestibility of protein.