Various investigators have observed significant effects of highly diluted histamine on human basophil degranulation in vitro compared to corresponding water controls. However, active and inactive dilution levels differed in most studies. Swiss researchers aimed to reproduce former studies with flow cytometry using rigorously controlled experimental conditions to minimise confounding factors. In seven independent experiments, basophils of the same human donor were incubated with diluted histamine (up to 10⁻³⁴ M) or water controls and activated with anti-IgE antibodies. Basophil activation was determined using bicolour flow cytometry. Experiments were blinded and performed with a randomised arrangement of the solutions on microtitre plates. Histamine at dilutions of 10⁻² and 10⁻²² M was associated with a significant inhibition of basophil degranulations of 23.1 and 5.7%, respectively, if compared to ‘diluted’ water treated in an identical manner. However, if all controls were pooled, only histamine 10⁻² M had a significant effect. Significant effects were seen for row numbers of the microtitre plates. The researchers were not able to confirm the previously reported large effects of homoeopathic histamine dilutions on the basophil function of the examined donor. Seemingly, minor variables of the experimental set-up can lead to significant differences in the results if not properly controlled.