Nimba arishta is an ayurvedic drug obtained by fermentation of a decoction of Azadirachta indica bark to which sugar and a kalka have been added. Kalkas consist of various plant materials including flowers of Woodfordia fruticosa. The effects of the preparation process on immunomodulatory properties of Nimba arishta were investigated by means of model systems. On the basis of traditional applications of Nimba arista in 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 the kalka resulted in a marked increase. Inoculation with yeasts (Saccharomyces cerevisiae or Schizosaccharomyces pombe var. pombe) or microorganisms introduced through addition of the kalka did not affect the in vitro immunomodulatory activities of Nimba arista. Further investigations revealed that Woodfordia fruticosa is the kalka ingredient with the most pronounced biological activities. Although in quantitative terms the bark of Azadirachta indica strongly dominates in Nimba arishta preparation, it was shown by detailed model studies that the flowers of Woodfordia fruticosa were found to represent a major ingredient with respect to modulation of complement activity.