A previously reported² effect of mechanically agitated dilutions of antiserum raised against IgE was investigated using the loss of metachromatic staining properties of human basophil leukocytes as a model. A series of 24 blind experiments was performed in which we determined the number of toluidine blue-stainable basophils after incubating with vortexed or non-vortexed dilutions of anti-IgE. Tenfold serial dilutions were used, in the range 10²¹ to 10³⁰ (6.6×10^–26 to 6.6×10^–35 M anti-IgE). We found no evidence for a different effect of strongly agitated dilutions, compared to dilutions made with minimal physical agitation. In fact, in our hands no effect of extreme dilutions was shown at all. We conclude that the effect of extreme dilutions of anti-IgE, reported by Davenas et al.², needs further clarification and that in this process the reproducibility of results between experimenters should be carefully determined.