A streptomycin-like ether insoluble antibiotic substance is produced by subjecting the solids of a culture of Actinomyces griseus that has produced streptomycin to extraction with an aqueous acid, and, if desired, recovering the antibiotic substance in the extract. The extraction of the solids may be effected after or before their separation from the culture liquid, or at both stages. Thus the solids may first be separated, e.g. by centrifuging or filtration, and then extracted with an aqueous acid. Alternatively the whole culture, preferably after substantially maximum streptomycin production has been attained, may be acidified and the solid and liquid components intimately contacted, whereby the antibiotic substance in the solids is extracted into the streptomycin-containing liquid and enhances its activity. The residual solids from this latter process may, if desired, be subjected to a further treatment with an aqueous acid. The acid used may be inorganic or organic, sulphuric and citric acids being preferred, and the extraction may be effected at various pH values, e.g. 5.6 or 1.1, preferably between about 1 and 3, especially about 1.5-2. The time required for extraction decreases with rise in temperature, a short extraction (about 1/2 -3 hours) at a relatively high temperature (about 45 DEG -55 DEG C.) giving the best yields. In examples: (1) (a) an aqueous medium containing soya bean meal, dextrose, meat extract and sodium chloride is incubated with spores of A. griseus at 25 DEG C. and 10-15 lbs. pressure while stirring and passing a current of air; (b) the resulting culture is acidified to pH 3.5 with hydrochloric, agitated at 5 DEG C. and centrifuged, yielding a supernate of enhanced activity; (2) (a) the solid residue from (1) (b) is washed twice with distilled water and then extracted with N/10 hydrochloric acid adjusted to pH2 with sodium hydroxide, centrifuged and the process repeated, the two extracts being used as such (separately or mixed), or added to the liquid product of (1), or treated (together or separately) as follows: (b) the extract is neutralized and its active ingredient purified by adsorption on activated charcoal, elution with dilute hydrochloric acid at 70 DEG C., treatment of the eluate with phosphotungstic acid, decomposition of the precipitated phosphotungstate with barium hydroxide and, if desired, freeze-drying; further purification may be effected by reaction with picnic acid in aqueous solution, dissolving the picrate in acetone, passing the solution through an activated alumina chromatographic adsorption column, eluting the antibiotic-rich sections of the chromatogram with aqueous acetone and decomposing the eluate with sulphuric acid to obtain the sulphate of the antibiotic substance, which may be still further purified by reaction with a Reinecke salt, and decomposing the Reineckate with silver sulphate in aqueous solution; (3) (a) a medium as in (1) (a), but omitting the meat extract, is inoculated with spores and mycelium of A. griseus and similarly incubated; (b) a portion of the resulting culture is treated as in (1) (b) (at 4 DEG C.); (4) (a) the remainder of the product of (3) (a) is centrifuged and the sediment washed and then slurried with distilled water; (b) a portion of the slurry is adjusted to pH 2.08 with 5N hydrochloric acid, shaken at 24 DEG C. (the pH rising to 2.4) and then centrifuged; somewhat inferior results are obtained at pH 1.1-1.22, 3.1-3.9 or 5.0-5.4; (c) the sediment from (b) is again extracted with hydrochloric acid at pH 2.55 (or 5.62 or 1.88); (5) a further portion of the slurry of (4) (a) is adjusted to pH2 with 5N hydrochloric acid, stirred at 50 DEG C. and centrifuged; somewhat inferior results are obtained by longer extractions at 4 DEG C. or 37 DEG C.; (6) a further portion of the slurry of (4) (a) is adjusted to pH 2.15 with 5N sulphuric acid, stirred at 50 DEG C. and centrifuged; the sulphuric acid may be replaced by 40 per cent citric acid; (7) a medium as in (1) (a), except that the percentage of meat extract is halved, is incubated as in (3) (a), and the resulting culture is adjusted to pH 1.5 with 100 per cent sulphuric acid, agitated with the diatomaceous earth filter aid known by the Registered Trade Mark "Celite" and the activated charcoal known as "Nuchar," and centrifuged.