Maleimycin is a bicyclic maleimide antibiotic elaborated by Streptomyces showdoensis. The physical and chemical data show the structure for maleimycin to be 3aza-6-hydroxybicyclo[3.3.0]oct-Al iS-ene-2,4-dione. The nuclear magnetic resonance (nmr) spectra in DoO showed five welldefined resonances corresponding to the five nonexchangeable protons. The 13C spectra consisted of seven resonances: two resonances in the carbonyl region at -41.3 and -40.7 ppm, two resonances in the aromatic or unsaturated region at -29.3 and -23.7 ppm, and one resonance at +56.3 ppm During a study of showdomycin, a second ultraviolet absorbing compound was observed in the culture filtrates of Streptomyces showdoensis. This compound has been isolated and crystallized, and its physical and biological properties have been studied in much detail. By the use of H (proton) nuclear magnetic resonance (pmr) and 13C nmr (cmr), the resonances of the five nonexchangeable protons and the spectra of the seven carbons are described. It is now possible to report the structure of this new maleimide antibiotic as 3aza-6-hydroxybicyclo[3.3.0]oct-A15-ene-2,4-dione. The biochemical properties and biosynthesis are also described. Experimental ProcedureIsolation of Maleimycin. Cultures of S. showdoensis were grown for 42 hr as described by Elstner et al. (1973). The medium was filtered, passed through Dowex 50-X8 (H*) (20-50 mesh, 3 cm X 20 cm). The pH was adjusted to 4.5 (ammonium hydroxide), adsorbed on Norit A (10 g/L), and washed with 1 1. of water. Maleimycin was eluted batchwise with acetone-water (80:20, v/v) (three times, 500 ml each time). The eluent was concentrated under vacuum to 2 ml, spotted on four Whatman No. 3MM paper chromatograms t From the