a - Hydroxy - g - alkylmercapto - butyric acids are made by heating an amide of formula <FORM:0722024/IV (b)/1> where R is H or Me and R1 is H or an alkyl group of 1-12 carbon atoms, in a solution containing 5 to 40 per cent by weight of mineral acid to a temperature at above 60 DEG C. and preferably from 80 DEG C. to the boiling-point of the solution. Their ammonium salts may be formed by the addition of an alkaline earth metal hydroxide or carbonate, e.g. calcium hydroxide, to the reaction mixture to liberate ammonia from the ammonium salt of the mineral acid, e.g. ammonium sulphate, formed in the reaction, the ammonia then combining with the mercapto acid. Excess alkaline earth metal hydroxide yields the alkaline earth metal salt or a mixture of this salt with the ammonium salt. The acids (excluding the methyl mercapto butyric acid) and their ammonium, calcium, strontium, barium and magnesium salts are claimed as new compounds and the latter salts of the methyl-mercapto-acid are used in poultry or animal feeds (see Group I). Hydrolysis may be effected in the presence of inert organic diluents, e.g. carbon tetrachloride. Aqueous hydrolysis is preferred. In examples: (1) and (2) a -hydroxy-g -methyl-mercapto-butyramide is hydrolysed with sulphuric acid to the free mercapto acid and converted to mixtures of its calcium and ammonium salts by treatment with calcium carbonate or hydride, and in (2) the zinc salt of the acid is also prepared for identification purposes. The a - hydroxy - g - alkylmercapto - butyramides are made by the hydrolysis of the corresponding nitriles, c.f. Specification 722,087.ALSO:Poultry or animal feeds deficient in methionine promote the growth of poultry and four-legged non-ruminants e.g. swine, cats and dogs, when fortified with an ammonium, calcium, barium, strontium or magnesium salt of a -hydroxy-g -methylmercaptobutyric acid or mixtures thereof. Amounts of about 0.1-0.5 per cent. by weight of the feed are specified for poultry. Mixtures of the ammonium and calcium salts are preferred and example (3) illustrates the rapid growth of chicks fed on a basal diet containing protein, carbohydrates, vitamins and minerals and 0.2 per cent. of a mixture of about 3 parts of the calcium to one part of the ammonium salt. For the preparation of the salts (see Group IV (b)).