Three new imidazole alkaloids, leucettamines A [1] and B [2] and leucettamidine [3], have been isolated from the Palauan sponge Leucetta microraphis. Their structures were established on the basis of extensive spectral analyses. Leucettamine A showed potent leukotriene B4 receptor binding activity (Ki =1.3 µM), while leucettamine B was essentially inactive (Ki = 100 µM) and leucettamidine showed significant activity (Ki = 5.3 µM). With leucettamine A identified as a pure LTB4 receptor antagonist, a new structure lead is presented to inflammation therapy. © 1993, American Chemical Society. All rights reserved.