FIELD: biotechnology, organic chemistry, chemical technology. SUBSTANCE: invention relates to method for preparing 2-hydroxy-4- methylthiobutanoic acid and/or its ammonium salt and an aqueous solution of indicated acid and its salt. At the first stage biological material eliciting nitrilase activity is prepared. At the second stage this material is fixed. At the third stage 2-hydroxy-4- methylthiobutyronitrile is interacted with the fixed biological material preparing 2- hydroxy-4-methylthiobutyric acid ammonium salt. At the fourth stage, if necessary, salt prepared on the third stage is converted to the corresponding acid. At the fifth stage product prepared on third and fourth stages is concentrated. An aqueous solution contains mixture of indicated salt and acid where mass part of acid is 0.05-0.99. Method can be used for industrial scale with enhanced yield and without using solvents and accompanying preparing mineral salts. EFFECT: improved preparing method. 20 cl, 12 dwg, 4 tbl
