TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Composition: The additives contains the biomass formed during the fermentation of the cysteine-compound-producing microorganisms at 0-100%.Preferred Component: The additives contains chemical compound(s) from the glutathione, cystathionine, biotin, thiamin, liponic acid, coenzyme A and/or L-methionine.Preferred Process: The feed additive is produced by drying, spray drying, spray granulation and/or granulation. For the preparation of the fermentation liquor containing cysteine compounds, L-cysteine-producing bacteria, fungi or yeasts are cultivated in a fermentation medium. The process further comprises electrochemical reduction (electrolysis) of the L-cystine to L-cysteine in first and/or second steps; acidification with a concentrated mineral acid in first and/or second steps; addition of a reducing agent to first, second and/or third steps; use of a protecting gas in first second and/or third steps; addition of an oxidizing agent to first second and/or third steps; addition of one or more cysteine compounds from L-cysteine, L-cystine and/or thiazolidines to first second and/or third steps, the added amount of cysteine compound being such that the total concentration, optionally including its salts, in the animal-feed additive is 1-98 weight%; addition of auxiliary substances to first, second and/or third steps, for stabilization and increasing the storability, from silicas, silicates, stearates, meals, brans, cereal flours, flours; silicas, silicates, starches and sugars; or conversion of the substances into a form stable in the animal's stomach by coating with film-forming agents.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Material: The salts of the cysteine compounds are sodium, potassium, ammonium, magnesium or calcium salt. The mineral acid is sulfuric acid. The oxidizing agent is oxygen (O2) or hydrogen peroxide (H2O2). The protecting gas is nitrogen (N2).    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Material: The reducing agent is vitamin C, vitamin E, formic acid and/or their salts. Preferred Composition: The additive contains 1-98 wt.% of cysteine compound(s) from group L-cysteine, L-cystine and thiazolidines, optionally including their salts. The thiazolidine content is at least 0.001 (preferably at least 0.1) wt.% or at least traces.    TECHNOLOGY FOCUS - POLYMERS - Preferred Material: The film-forming agent is metal carbonates, silicas, silicates, alginates, stearates, starches, rubbers or cellulose ethers. SPECIFIC COMPOUNDS - The thiazolidines are 2-methyl-thiazolidine-2,4-dicarboxylic acid, 2-carboxymethyl-thiazolidine-2,4-dicarboxylic acid, 2-carboxyethyl-thiazolidine-2,4-dicarboxylic acid, or thiazolidine-2,4-dicarboxylic acid.    EXAMPLE - Fermentation liquor was centrifuged for 20 minutes in a centrifuge at 3500 rpm and at room temperature. Each of centrifuge liquor (1317 g) and the original fermentation liquor were mixed. The resulting 2634 g were then dried to 400 g in 4.5 hours in an evaporator. An L-cysteine concentration of 13.2 g/kg was determined in the resulting product. The concentration of L-cysteine was 9.7 g/kg. The concentration of the thiazolidine was estimated at 26 g/kg. The content of total dry mass was 49.7 weight%.