To study the time-course of eosinophil cationic protein (ECP) and eosinophil protein X (EPX) during bronchial allergen provocation, we investigated 32 asthmatic children sensitive to house-dust mites as well as 6 non-atopic young adult controls. In all subjects, allergen challenges were performed with house dust mite extracts of Dermatophagoides pteronys-sinus or Dermatophagoides farinae. Blood samples were taken at regular intervals during the 24-h observation period. The individual time-courses of ECP and EPX revealed different characteristic groups of patterns: (1) an isolated early serum peak of both mediators during or within the first 60 min after provocation (2) an early plus a late peak (3) an isolated late peak 12 h after provocation (4) an isolated late peak 24 h after provocation, and (5) no significant variation during the 24-h observation period. The early peak could be due to short-term changes in eosinophil activation, while late peaks may reflect eosinophil proliferation, recruitment, subsequent priming and enhancing of the propensity to release their proteins. ECP and EPX showed a corresponding parallel time-course in nearly all challenges, with EPX-concentration exceeding that of ECP. There was no correlation between ECP/EPX serum concentrations and clinical parameters such as lung function data. From our results we conclude that the striking groups of time-courses of ECP/EPX serum concentration indicate different uniform patterns of eosinophil activation during allergen challenge-but do not predict clinical outcome of provocation. The role of the eosinophil in early asthmatic reactions remains to be established in further studies.