To determine whether the measurement of specific markers of inflammatory cells in peripheral blood might be used to detect the inflammatory activity in the airways in asthma induced by toluene diisocyanate (TDI). we measured the levels of eosinophil cationic protein (ECP), histamine and tryplase in peripheral blood before and during inhalation challenge with TDI or methacholine in two groups of subjects who exhibited or did not exhibit an asthmatic reaction after exposure to toluene diisocyanate in the laboratory. When the subjects developed a late asthmatic reaction after exposure to TDI, the\ showed an increase in their ECP serum levels. By contrast, there were no signilicam changes in serum ECP levels after exposure to TDI in the control group or after methacholine challenge in either group. Tryptase levels in serum were not detectable before or during inhalation challenge with TDI or methacholinc. There was no significant increase in plasma histamine levels during inhalation challenge with TDJ or methacholine. These results suggest that eosinophils arc ‘activated’ in subjects who develop a late asthmatic reaction after exposure to TDI and that the measurement of ECP levels in peripheral blood may be a useful marker to monitor airway inflammation.