The novel alkaloid halichlorine was isolated from Halichondria okadai Kadota and its structure was elucidated by spectroscopic analysis. The relative stereochemistry was clarified by a detailed analysis of coupling constants and the NOESY spectrum. Halichlorine significantly inhibits VCAM-1 induction in cultured HUVE (human umbilical vein endothelial) cells. Copyright (C) 1996 Elsevier Science Ltd