A multicentre collaborative clinical trial has been made at one centre in each of six countries to decide if the potency of allergen extracts can be determined satisfactorily by skin-prick tests in man.

Although there was considerable variation in potency of antigen when determined in different patients, if a sufficiently large group (n=54) of persons were tested, it was shown to be feasible to compare potencies of sequential batches of the same specificity, and also of antigens of different specificities.

It was shown that batches of mite extract were weaker than that of a grass pollen, but it proved to be a simple matter to adjust the concentration to give the same biological potency.