A bright blue sponge of the genus Reniera, which has been found both at Isla Grande, Mexico, and some 14 200 km distant, in an obscure marine lake in Palau, Western Caroline Islands, was shown to contain a series of alkaloids that included renierone (l), mimosamycin (2), N-formyl-1,2-dihydrorenierone (3), the isoindole 4, and renieramycins A-D (5-8). A similar suite of metabolites that include mimosamycin (2), mimocin (9), and saframycins A (lo), B (ll), C (12), and S (13) were isolated after treatment of the culture medium of a streptothricin-producing strain of Streptomyces lavendulae no. 314 with sodium cyanide. Striking similarities between renierone (1) and mimocin (9) and between the renieramycins 5-8 and saframycins 10-13 were noted. However, the stereochemistry assigned to the renieramycins 5-8 differed from that determined by X-ray analysis of saframycin C (12) at C-1, the point of attachment of the side chain. In this paper we report the isolation of two very unstable compounds, renieramycins E (14) and F (15), and propose that the structures of renieramycins A-D (5-8) be reassigned to 16-19, in which the stereochemistry is identical with that of the saframycins. The sponge Reniera sp. was recollected in January 1988 from the same marine lake at Palau. After extraction with methanol and chromatography (silica gel, Sephadex LH-20, HPLC), renieramycins E (14) and F (15) were isolated. Renieramycin E (14) has the same molecular formula as renieramycin A (16) (C30H34N2O9), and its structure was determined by spectral data (HRMS, IR, UV, NMR) and comparison with renieramycin A. Renieramycin F (15) has a molecular formula of C31H36N2O10, and its structure was deduced by spectral comparison with renieramycins E and B. NMR experiments (coupling constants, NOE enhancements) indicated that the stereochemistry of renieramycins E and F is identical with that of the saframycins. Renieramycins E and F are very unstable, decomposing to renierone (1) and mimosamycin (2), suggesting oxidative cleavage. The stereochemistry of all renieramycins is now believed to be the same as that of saframycin C, which was determined by X-ray analysis, as previous NOEDS experiments on renieramycins A-D were performed with too high a power level leading to flawed results.