The concentrations of the eosinophil (EOS)-derived proteins, major basic protein (MBP), EOS-derived neurotoxin (EDN), EOS peroxidase (EPO), and EOS cationic protein (ECP), and EOS counts were measured in the peripheral blood of 12 atopic subjects with asthma and 23 normal control subjects. The same measurements were performed in seven subjects with asthma with previously documented dual (early plus late) asthmatic responses after inhalation challenges with methacholine and allergen. EOSs (p less than 0.001), MBP (p less than 0.01), EDN (p less than 0.01), and ECP (p greater than 0.03) were elevated in the subjects with asthma compared with control subjects, whereas EPO (p less than 0.01) concentrations were reduced. There were no significant differences between baseline measurements of FEV1, EOSs, MBP, EDN, EPO, or ECP on the methacholine- and allergen-challenge days. When the changes in these variables after allergen challenge were compared with the corresponding changes after methacholine challenge, there were no significant differences at 0 to 60 minutes or at 3 hours, whereas EDN (p less than 0.025), EPO (p less than 0.05), and ECP (p less than 0.025) were relatively increased at 6 to 12 hours and accompanied the late falls in FEV1 (p less than 0.001). EOSs (p less than 0.025) were elevated at 24 hours when there was a small relative increase in MBP (p less than 0.05). EOSs appear to be "activated" in subjects with allergic asthma, and further activation may occur during late asthmatic responses.