Relevance. The aim of the work was to determine the amino acid composition of the culture filtrates of the strains, the causative agent of flax anthracnose Colletotrichum lini Manns et Bolley, to adjust the concentration of the selective agent in the nutrient medium when creating in vitro new flax genotypes resistant to anthracnose. Results. It was established that in the culture filtrates of strains 527 and 608 there are amino acids alanine, glycine, asparagine, cysteine, threonine, aspartic acid, glutamic acid, as well as arginine in the highly virulent strain 527 and traces of tyrosine and lysine in the weakly virulent strain 608. K 40 days of cultivation, the supply of nutrients in the cultivation medium, apparently, was exhausted, and for life support the fungus began to use the products of its vital functions. In the culture filtrate of the highly virulent strain 527, the concentration of all defined amino acids was significantly higher than in the culture filtrate of the weakly virulent 608 strain. It was shown that the 23-day-old culture filtrate of the strongly virulent strain 527 had the highest toxicity. The increase in the roots and hypocotaly of flax when using the culture filtrate of the strongly virulent strain 527 was smaller in all genotypes taken in the study. The toxicity of the culture filtrate depended on the virulence of the anthracnose pathogen strain — the culture filtrate of a strongly virulent strain is more toxic than a weakly virulent strain. The presence of cysteine in the culture filtrates of the strains increases the possibility of inhibiting the growth and development of flax cells in an in vitro culture. When using a culture filtrate of anthracnose pathogen strains containing asparagine, glutamine, serine, glycine, aspartic and glutamic acid, it is possible to induce the growth and development of flax cells in vitro. As the fungal mycelium grew in the culture filtrates, the concentrations of amino acids alanine, asparagine, glycine, aspartic and glutamic acids decreased. Due to the high concentration of cysteine and tyrosine, the culture filtrates of strains 419 and 639 were toxic throughout the study period (up to 42 days).