Asparagus-P® is a traditional herbal medicinal product consisting of a combination of asparagus roots and parsley leaves in equal shares. It is used to support aquaretic kidney function. The present study was undertaken to shed light on the antioxidative effects of both main ingredients and their combination as in Asparagus-P®. For that purpose, differentiated promyelocytic HL60 cells (functional neutrophils) capable of generating superoxide radicals upon stimulation, were used. Parsley leaves and, hence, also Asparagus-P® had a marked dose-dependent antioxidant effect, whereas asparagus roots had only a minor efficacy in this respect. In conclusion, Asparagus-P® does not only support kidney function, but is also able to inactivate reactive oxygen radicals which might occur by a metabolic or exogenous overload (oxidative stress) or in the course of inflammatory processes. This antioxidant efficacy is primarily related to the content of parsley leaves; asparagus roots seem to enhance this efficacy by promoting the potential of parsley leaves, thus generating a synergistic effect in the combination product Asparagus-P®. Copyright © 2008 John Wiley & Sons, Ltd.