Eleven ent-kauranes, isolated from the fresh stems of Annona squamosa L. (Annonaceae), were subjected to assays on the generation of superoxide anion (O2.--) by human neutrophils. Except for ent-kaur-16-en-19-oic acid, 16beta,17-dihydroxy- ent-kauran-19-al, and 16alpha,17-dihydroxy -ent-kauran-19-al, all ent-kauranes showed significant inhibitory effect on O2.-- generation in response to formyl- L-methionyl- L-leucyl- L-phenylalanine (fMLP/CB). In contrast, phorbol myristate acetate (PMA)-induced O2.-- generation was not suppressed by any ent-kauranes. Especially, ent-kaur-16-en-19-oic acid could significantly increase O2.-- production. The structure-activity relationship of these compounds is also discussed herein. Furthermore, the effect of ent-kauranes on nitric oxide generation by NR8383 macrophages in response to lipopolysaccharide (LPS) was examined. None of the compounds showed an inhibitory effect on nitric oxide generation.