We have studied a number of aspects of the interrelationship between T lymphocytes and allergic inflammation. Using monoclonal antibodies and immunocytochemistry we have observed significant infiltration by CD3/VCD4+ T lymphocytes together with evidence of T cell activation in the allergen-induced late-phase skin reaction in human atopic subjects. These changes were accompanied by eosinophil infiltration and activation. Activated T cells were also detected in the blood of patients with acute severe asthma. The role of T lymphocytes in granulocyte chemotaxis and activation was also explored, using recombinant human lympho-kines, and we have identified and characterized two lymphokines which have specific effects upon neutrophils.