The aim of the study was to determine the effects of different proportions of Medicago media Pers. in mixtures with Festulolium braunii (K. Richt.) A. Camuson the yield of aboveground biomass, protein yield and relative chlorophyll content in festulolium leaves. The study was carried out in 2011-2013. A strict field experiment was conducted at the Educational and Research Station of the University of Warmia and Mazury in Olsztyn (Poland), on mineral soil of quality class IVa and very good rye complex. Pure-sown festulolium and festulolium in mixtures with hybrid alfalfa were investigated. The experimental factor was a varied proportion (30, 50, 70%) of alfalfa seeds in the mixtures. The results demonstrated that the proportion of alfalfa in mixtures increased gradually in subsequent years, but its share in the aboveground biomass was greater to the percentage of seeds sown. Alfalfa introduced as a component of the mixtures increased the relative chlorophyll content in festulolium leaves, but no significant differences were found between plots with 30 and 50% proportions of alfalfa. The mixtures gave significantly higher yields when compared to pure-sown festulolium. Depending on the proportion of alfalfa in the mixtures, the yield was about 40 to 46% higher and more reliable in the study years. The introduction of alfalfa to the mixtures caused a significant 61-80% increase in protein yield per area unit as compared to pure-sown festulolium. The high values of the correlation coefficients demonstrate that the leaf greenness index is an important predictor of plant yield and protein yield.