In modern pharmaceutical practice, it is common to use purified ethanol and purified water for the preparation of homeopathic dilutions. Hahnemann in 1827 recommended good brandy as a diluent. Brandy contains a lot of accompanying substances in addition to ethanol. The research question was whether different diluents influence the effectiveness of high dilutions, especially above Avogadro's number. We compared two dilution media to investigate the diluent's influence. Within the limitations of the test-system, the dilution media were as similar to good brandy as possible and like purified ethanol. Dilutions of histamine were prepared with both media. As test-system, we used modified basophil activation in an in vitro cell system. Basophils are activated by anti-immunoglobulin E (anti-IgE). The activation of basophils is inhibited by prior incubation with histamine. The reduction in activation was measured with different dilutions of histamine. The test system used a 3-colour flow cytometric method. The interleukin-3 (IL-3) receptor CD123 was used to identify basophils in the leukocyte mixture. The CD63 surface marker was used for quantification of activated basophils. With higher concentrations of histamine, we observed inhibition on optimally anti-IgE-stimulated basophil activation with a clear concentration dependency. With low concentrations of histamine (up to 10(-31)), we also observed inhibition of IgE-mediated basophil activation. Differences were observed between the dilution media. The preliminary results support the hypothesis that the dilution medium may influence the effects of high dilutions. This could be of importance for homeopathic pharmaceutical practice as well as for ultra-high dilution experiments. The refined basophil test system proved to be highly sensitive and reliable. Further studies are needed.