Today, plant waste, including fallen poplar leaves (litter), are a promising raw material for the production of useful products using bioconversion, such as protein feed additives. The aim of this study was to study the component composition of the products obtained as a result of the bioconversion of leaf litter. The strain PP-3.2 Pleurotus pulmonarius (Fr.) Quél. In the process of substrate conversion, strain PP-3.2 primarily utilizes extractives and easily hydrolyzable polysaccharides, the amount of which decreases by 44 and 36%, respectively. The total content of polysaccharides is reduced by 20%, lignin substances - 9.4%. The loss of substrate mass based on leaf litter was 23%. The product obtained after cultivation contains 28% protein. Analysis of the amino acid composition of the protein showed a high rate of phenylalanine with tyrosine (115), threonine (117.5), valine (110) and isoleucine (105%). Also, the biodegradation product has a high content of glutamic and aspartic acids (11.2 and 25.3% of the total amino acids, respectively). The levels of cadmium, lead and copper revealed as a result of studies do not exceed the maximum permissible concentrations established by the veterinary department. At the same time, there is a high content of iron (166.0 μg/kg) and zinc (256.7 mg/kg) in the feed product. The digestibility of the product is 54%, the content of nucleic substances is up to 0.3%. Thus, the results obtained show the possibility of using the product of the conversion of poplar fallen leaves as a protein feed additive.