Keramamine-A and -B, two new B-carboline alkaloids with a 13 membered ring have been isolated from the Okinawan marine sponge Pellina sp. as antimicrobial substances and the structures of keramamine-A and -B have been determined by X-ray analysis and by spectral and chemical means, respectively. As part of our research program on physiologically active marine natural products, we have examined pharmacological effects of extracts of various marine organisms. 2 Marine sponges are one of good soruces for bioactive metabolites with unusual structures. 3 In this paper, we report the isolation and structural elucidation of keramamine-A and -B, novel alkaloids with antimicrobial activities from the Okinawan marine sponge Pellina sp. The brownish sponge Pellina sp. was collected at Kerama Islands using SCUBA. The ethanolic extracts were partitioned between hexane and 80% ethanol. Chloroform soluble part of the 80% ethanol portion was chromatographed twice on Sephadex LH-20 columns (chloroform-methanol 1:l and ethanol), followed by a silica gel column (chloroform-methanol 98:2) to give keramamine-A (1) hydrochloride as a pale yellow solid (0.026% from wet sponge) and crude keramamine-B, which was further purified by silica gel column chromatography (I-butanol-acetic acid-water 4:l:l and chloroformmethanol 95:5) to give an acetic acid salt of keramamine-B (2) as a yellow solid (243 mg, 0.048%). Keramamine-B (2) hydrochloride was obtained by partitioning it between chloroform and a saturated aqueous NaCl solution. Keramamine-A (1) hydrochloride,4 [a]:' +44.3" (c 1.09, CHC13), decomposed at 2OO"C without melting and showed W absorption maxima at 238 (E 15300), 278 (9680), 290 (8350), 346 (4420) and 358 nm (4770), suggesting the presence of a 8-carboline chromophore. 5 The molecular formula C36H440N4 for 1 was determined by HREIMS (m/z 548.3495, M+, A-2.0 mmu). The structure of 1 was established by X-ray analysis data obtained for a single crystal of the hydrochloride (MeOH) as illustrated in Figure 16 . The 'H and 13C NMR signals for 1 were analyzed by two dimensional 'H-'H and 'H-13C shift correlation experiments, and quaternary carbon signals were assigned by 'H-13C shift correlation via long range couplings by COLOC' as shown in Table 1.