SummaryWe report the finding of anomalous responses in the skin of atopic individuals when prick tested with serial dilutions of cocksfoot grass pollen extracts. Each pollen extract was standardized using RAST inhibition activity related to the WHO standard for IgE. 75% of the patients gave a larger weal area with an allergen concentration of 1:105than with the next higher concentration, i.e. 1:104.Possible mechanisms whereby a larger skin weal is elicited with a lower concentration of allergen are discussed. This is not a rare finding and can be observed in patients undergoing routine skin testing in the clinic, even where extracts are diluted over a relatively narrow concentration range.