The possible mode of action of the recently demonstrated steroid effect on the immediate type allergic reaction has been studied. The influence of a topical steroid, budesonide, on the nasal mucosal histamine content and anti-IgE induced histamine release was studied in an open study. A 1-week treatment with budesonide, used locally in the nose, was administered to 22 hay fever patients who were studied out of the pollen season. There was a decrease of histamine content after steroid treatment and also a blockade of the anti-IgE mediated histamine release, as shown in an in vitro release procedure. This steroid effect may partly explain the effect of steroids on the immediate reaction, as it has been demonstrated in allergen challenge studies.