Tomaymycin, a new antiphage antibiotic is produced by cultivation of Streptomyces achromogenes var. tomaymyceticus in a lactose-bouillon medium enriched with yeast extract and phosphate salts. The active substance can be extracted from the culture filtrate by carbon adsorption and further purified by silica column chromatography. Tomaymycin is obtained in crystalline form as a molecular complex with methanol. The complex occurs as colorless platelets, m.p. 145-146°C, [α]20D + 423° (c 0.5, pyridine), λmax 224, 237, 260, 320 mμ (in methanol). The molecular formula is C16H20O4N2. The antiphag°e activity is strong against Escherichia coli T1, T3 phages and Bacillus subtilis M-2, SP-10 phages. Tomaymycin has also antimicrobial activity against Grampositive bacteria. The LD50 (i.p.) of tomaymycin for mice is estimated at 3.0 mg/kg.