We studied time course of the blood peptide LT, LTB₄ and the neutrophil count in blood following food challenge in children with food allergy. Of 13 challenges, 11 challenges induced immediate reactions. The mean blood peptide LT and LTB₄ levels rose significantly at 1 hour after challenge when immediate reactions began to appear. At 2 and 3 hours the levels decreased and they rose again at 4 hours when symptoms continued. The mean neutropil counts in blood began to rise significantly at 2 hours after challenge later than the first peak of LT level and showed the peak at 4 hours. Two challenges induced late reactions. Blood peptide LT levels slightly rose at 1 hour and 4 hours after challenge. Blood LTB₄ levels rose at 6 hours after challenge before late reactions began to appear. The neutropil counts in blood did not rise. This study suggests the elevation of LT may be associated with the appearance and duration of the allergic reaction.