6-Dimethyl tetracycline antibiotic producing gene-recombinants of a species of the genus Streptomyces are obtained by subjecting two different strains of a tetracycline antibiotic producing species of Streptomyces, one of which strains is deficient in its methionine biosynthesis, to gene-recombination in the presence of a folic acid antagonist and selecting and isolating a 6-dimethyl tetracycline-producing strain from the resulting culture.  The strain or strains other than the methionine deficient strains may exhibit normal biosynthetic processes or be deficient in a biosynthetic substance other than methionine.  The methionine deficient strain may exhibit an additional biosynthetic deficiency.  The folic acid antagonist includes 4-amino-pteroyl-glutamic acid, 4-amino-10-methyl-pteroyl glutamic acid and sulphonamides of formula:- <FORM:1091045/C6-C7/1> where R is a substituted or unsubstituted guanidine, thiazole, pyrimidine or pyrazino radical. The gene-recombination may be effected in generally known manner in a "minimal medium" to which is added the substances in which the strains are deficient in their biosynthesis.  In examples, gene-recombinants occur when the two mutants are as follows:-  (a) Streptomyces aureofaciens methionine deficient with S. aureofaciens arginine and cysteine deficient.  (b) S. aureofaciens methionine and amino nitrogen deficient with S. aureofaciens methionine and asparagine deficient.  (c) S. aureofaciens methione and asparagine deficient with S. aureofaciens arginine and cysteine deficient.ALSO:6 - Demethyltetracycline - antibiotic - producing gene-recombinants of a species of the genus Streptomyces are obtained by subjecting two different strains of a tetracycline-antibiotic-producing species of Streptomyces, one of which strains is deficient in its methionine biosynthesis to gene recombination in the presence of a folic acid antagonist, and selecting and isolating a 6-demethyl-tetracycline-producing strain from the resulting culture. The strain or strains other than the methionine-deficient strain may exhibit normal biosynthetic processes or be deficient in a biosynthetic substance other than methionine.  The methionine-deficient strain may exhibit an additional biosynthetic deficiency.  The folic acid antagonist includes 4-amino-pteroylglutamic acid, 4-amino - 10 - methyl - pteroylglutamic acid and sulphonamides of formula <FORM:1091045/C2/1> wherein R is a substituted or unsubstituted guanidine, thiazole, pyrimidine or pyrazino radical.  The gene-recombination may be effected in generally known manner in a "minimal medium" to which is added the substances in which the strains are deficient in their biosynthesis. In examples, gene-recombinants occur when the two mutants are as follows: (a) Streptomyces aureofaciens methionine-deficient, with S. aureofaciens arginine- and cysteine-deficient, (b) S. aureofaciens methionine-and amino-nitrogen-deficient with S. aureofaciens cysteine- and asparagine-deficient, (e) S. aureofaciens methionine- and asparagine-deficient with S. aureofaciens arginine- and cysteine-deficient.  A 6-demethyltetracycline antibiotic is produced by culturing in a conventional manner a gene-recombinant as above or a descendant or mutant thereof.  Chloride ions, bromide ions or chlorination or bromination inhibitors may be present to produce predominantly the appropriate chlorotetracycline, bromotetracycline or tetracycline.  A methylation inhibitor may be added gradually in accordance with Specification 1,091,046.