The objective of the research (2004 – 2006) was to determine the yield of seeds, straw, crop residues, and total protein in mixtures of oats and narrow-leaved lupine depending on the content proportions of those species and to compare them with the relevant indicators of the named plants originating from pure stands. A field experiment was established at the Experimental Farm in Zawady, and arranged using a randomized complete block design with four replicates. The experimental factor was a content proportion (% of pure stand sowing) of oats (100, 75, 50, 25, 0) and narrow-leaved lupine (0, 25, 50, 75, 100) in a mixture. Of the oats/narrow-leaved lupine mixtures studied, the mixture with a 75% and 25% proportion of components produced the highest yields of seed, straw, crop residue, and total protein. The yields of this mixture were similar to those of oats cultivated in pure stands. An decrease in the content proportion of oats in the mixture with narrow-leaved lupine caused the yields of seed, straw, and crop residue to significantly decrease, and the total protein content in the seeds and straw of mixtures to increase. The per cent proportion of oats in the structure of mixture yield was between 88.8 and 96.0%. When cultivating mixtures of oats and narrow-leaved lupine, it is possible to produce significantly higher yields of seed, straw, crop residue, and total protein if compared with the yields obtained when growing narrow-leaved lupine in pure stands.