The ansamycins constitute an important class of antibiotics which includes the clinically useful rifamycins and numerous other compounds as well as the streptovaricins. Previous extensive investigations of the components of streptovaricin complex included the protostreptovaricins (1_5) and damavaricins (6, 7), presumed biosynthetic precursors of the streptovaricins. This report describes a novel member of this class, streptovaricin U, in which the characteristic ansa chain of the ansamycins has been cleaved. Streptovaricin complex was chromatographed four times over silica gel (using various chloroform-methanol-water and ethyl acetate-ethanol-water mixtures as eluants) and finally subjected to countercurrent distribution (hexane-ethyl acetate-95% ethanol-water = 1:1:1:1, 500 transfers) to obtain streptovaricin U [0.6% yield from complex, C₃₆H₄₉NO₁₀ (hydrate of protostreptovaricin I), orange powder, mp 115-140°C, [α]ᴅ -110.2° (c 1, MeOH)]. Hydrolysis of streptovaricin U in refluxing methanolic sodium hydroxide, followed by purification and sublimation, gave 2-amino-3,7-dimethyl-6-hydroxy-1,4-naphthoquinone (8), whose structure was confirmed by methylation (to 9), ¹H NMR (showing olefinic/aromatic methyl groups, aromatic protons, and NH₂ group), and mass spectrometry (major C₇H₆O ion from quinone ring loss). Spin decoupling of the ¹H NMR spectrum (C₅D₅N) and ¹³C NMR analysis confirmed that streptovaricin U (assigned structure 10) is hydrolyzed protostreptovaricin I, with C-17 as a carboxyl group—this was further confirmed by converting streptovaricin U to its methyl ester (11) and 19-O-methyl methyl ester (12) (showing an infrared band at 1710 cm⁻¹ for unsaturated ester). While the question of whether streptovaricin U is an artifact of the work-up procedure remains unresolved, enzymatic hydrolysis is favored because isolation of streptovaricin complex does not involve base, and protostreptovaricin I treated with methanolic hydroxide (under conditions that produce 8 from 10) did not form 8 or 10. Streptovaricin U, the first open-chain "ansamycin", has no antimicrobial or L1210 inhibitory activity but inhibits RLV RDDP (Rauscher leukemia virus RNA-dependent DNA polymerase) by 40% at 200 μg/ml, demonstrating that the ansa ring is unnecessary for reverse transcriptase inhibition.