Objective: Previous research suggests that human basophil activation may be inhibited by histamine even at extremely low doses (high dilutions). However, uncertainties about the nature of the phenomenon and its reproducibility mean that further, rigorously controlled studies are necessary.Methods: Serial 1:100 (v:v) histamine dilutions (centesimal dilutions, C) and water controls were tested on human basophil responsiveness to anti-IgE antibodies, using flow cytometry. Each dilution step was followed by vertical mechanical shaking (also designed as succussion) at 20 strokes/s. Basophil-enriched buffy coats from healthy blood donors were incubated with 10−4 mol/l histamine (2C) and with serially diluted preparations from 10−20 mol/l (10C) to 10−32 mol/l (16C), then incubated for 30 min with 1 μg/ml goat monoclonal anti-human IgE and basophils stained for immunophenotyping.Results: Membrane up-regulation of CD203c, which in these experimental conditions proved to be a more consistent activation marker than CD63, was significantly inhibited in samples treated with histamine at the dilutions of 2C (P = 0.001), 12C (P = 0.047), 14C (P = 0.003), 15C (P = 0.036) and 16C (P = 0.009). Control water dilutions/succussions did not show any significant effect.Conclusion: Using a strictly standardized flow cytometry protocol and a new dilution/succussion procedure, we have shown that low and high dilutions of histamine inhibit CD203c up-regulation in anti-IgE stimulated basophils.