Macrosphelide L has been isolated from a strain of Periconia byssoides originally separated from the sea hare Aplysia kurodai, and the absolute stereostructures of this material and macrosphelide H, previously undetermined, have been elucidated on the basis of spectroscopic analyses using 1D and 2D NMR techniques and some chemical transformation.(R)-Methyl 3-p-bromobenzoyloxy-5-oxohexanoate has been synthesized for configurational assignments of macrosphelide H. These macrosphelides inhibited the adhesion of humanleukemia HL-60 cells to HUVEC.Based on the fact that some of the bioactive materials isolated from marine animals have been produced by bacteria, we have focused our attention on new antitumour materials from microorganisms separated from marine organisms.1-3) As part of this study, we have previously isolated the five 16-membered macrolides, macrosphelides E (1)-H (2) and I, from a strain of Periconia byssoides OUPS-N133 originally separated from the sea hare Aplysia kurodai, and reported their absolute stereostructures except for macrosphelide H (2).4,5) Although the planar structure for 2 was previously reported,4,5) its relative and absolute stereochemistries were undetermined. All of these compounds except for macrosphelide I inhibited the adhesion of human-leukemia HL-60 cells to humanumbilical-vein endothelial cells (HUVEC) more potently than herbimycin A,6,7) and compound 2 exhibited most potently inhibitory activity.5) Further investigation for metabolites of this fungal strain has now led to the isolation of one additional new anti-adhesion compound designated macrosphelide L (3). It has been reported by OMURA and co-workers that macrosphelides A-D, J and K have been isolated from a strain of Microsphaeropsis sp. and the first four compounds potently inhibit cell-cell adhesion.8-12)We describe herein the absolute stereostructures of macrosphelides H (2) and L (3), and the inhibition of cell adhesion for the latter and some derivatives (Fig. 1).Results and Discussion medium containing malt extract 1%, glucose 1% and peptone 0.05% in artificial seawater adjusted to pH 7.5, as reported previously.4,5) The AcOEt extract of the culture filtrate was purified by fractionation employing a combination of Sephadex LH-20 and silica gel column chromatographies and HPLC to afford macrosphelide L (3) together with macrosphelide H (2).Macrosphelide L (3) had the molecular formula C16H22O8 established by HREIMS. Its IR spectrum exhibited bands at 3438, 1718, 1710 and 1651cm-1, characteristic of an alcohol, an ester, a ketone and a double bond (Table 1). A close inspection of the 1H and 13C NMR spectra of 3 (Table 2) by DEPT and 1H-13C COSY