AbstractNimba arishtais an ayurvedic drug obtained by fermentation of a decoction ofAzadirachta indicabark to which sugar and akalkahave been added.Kalkasconsist of various plant materials including flowers ofWoodfordia fruticosa. The effects of the preparation process on immunomodulatory properties ofNimba arishtawere investigated by means of model systems. On the basis of traditional applications ofNimba aristain Sri Lanka, complement activity and functioning of granulocytes were selected as relevant immunological parameters in our studies. Long boiling times, as a consequence of decoction preparation on a commercial scale, were found to substantially reduce the immunomodulatory activities, whereas addition of thekalkaresulted in a marked increase. Inoculation with yeasts (Saccharomyces cerevisiaeorSchizosaccharomyces pombevar.pombe) or microorganisms introduced through addition of thekalkadid not affect thein vitroimmunomodulatory activities ofNimba arista. Further investigations revealed thatWoodfordia fruticosais thekalkaingredient with the most pronounced biological activities. Although in quantitative terms the bark ofAzadirachta indicastrongly dominates inNimba arishtapreparation, it was shown by detailed model studies that the flowers ofWoodfordia fruticosawere found to represent a major ingredient with respect to modulation of complement activity.