Thirteen allergic asthmatic patients were challenged six times each and serum levels of ECP (eosinophil cationic protein), EPX (eosinophil protein-x) and blood counts of eosinophil granulocytes were measured in blood obtained before and at regular intervals after challenge. Three challenges were performed in a blinded and randomized fashion and included a one-dose pretreatment with the inhalant anti-asthmatic drugs disodium cromoglycate, terbutaline and budesonide. One challenge was performed after 4 weeks' pretreatment with the inhalant budesonide and one was a histamine challenge. Pre-challenge levels of ECP were significantly reduced both after 4 weeks and after a one-dose treatment with budesonide whereas the EPX levels were reduced only after the former. Blood eosinophil counts were unaffected by the challenge whereas the ECP levels were significantly reduced after the placebo challenge and when premedicated with disodium cromoglycate and terbutaline. The EPX levels stayed unaltered at the placebo challenge but were significantly reduced when the patients were premedicated with terbutaline. ECP and EPX levels as well as blood eosinophil counts before challenge were significantly related to the development of the late asthmatic reaction. The results again focus on a relation between the eosinophil granulocyte and asthma and suggest that an increased activity of the blood eosinophil, as suggested by the raised serum levels of the granule proteins ECP and EPX, is one prerequisite for the development of chronic asthma.