Nimbidin is a mixture of tetranortriterpenes and is the major active principle of the seed oil of Azadirachta indica A. Juss (Meliaceae) possessing potent antiinflammatory and antiarthritic activities. The present study revealed that nimbidin significantly inhibited some of the functions of macrophages and neutrophils relevant to the inflammatory response following both in vivo and in vitro exposure. Oral administration of 5-25 mg/kg nimbidin to rats for 3 consecutive days significantly inhibited the migration of macrophages to their peritoneal cavities in response to inflammatory stimuli and also inhibited phagocytosis and phorbol-12-myristate-13-acetate (PMA) stimulated respiratory burst in these cells. In vitro exposure of rat peritoneal macrophages to nimbidin also inhibited phagocytosis and PMA stimulated respiratory burst in these cells. Nimbidin also inhibited nitric oxide (NO) and prostaglandin E 2 (PGE2) production in lipopolysaccharide (LPS) stimulated macrophages following in vitro exposure, whereas interleukin 1 (IL-1) was only weakly inhibited. Probing the mechanism of NO inhibition revealed that nimbidin ameliorated the induction of inducible NO synthase (iNOS) without any inhibition in its catalytic activity. In addition, nimbidin also attenuated degranulation in neutrophils assessed in terms of release of β-glucuronidase, myeloperoxidase and lysozyme. The results suggest that nimbidin suppresses the functions of macrophages and neutrophils relevant to inflammation. Thus nimbidin can be valuable in treating inflammation/inflammatory diseases. Copyright © 2004 John Wiley & Sons, Ltd.