T o examine the role of adhesion molecules in T cell recruitment and activation during allergen-induced late asthmatic response (LAR), we evaluated the expression of lymphocyte function-associated antigen-1 alpha (LFA-1 alpha) and intercellular adhesion molecule-1 (ICAM-1) on peripheral blood T lymphocyte subsets from atopic asthmatic patients and their changes following allergen inhalation challenge. 12 atopic asthmatic patients were studied. Six patients showed only a single early response after allergen challenge, and six developed a dual response. At baseline, dual responders (DR) had a significantly higher expression of ICAM-1 on CD4+ and CD8+ T lymphocytes as compared with both single early responders (P < 0.005 and P < 0.02, respectively) and controls (P < 0.001, both comparisons). Allergen challenge was followed by a decrease of CD8+ ICAM-1+ T lymphocytes in all DR (P < 0.05) and of CD4+ ICAM-1+ T lymphocytes in four out of six DR, at the time of the LAR. At the same time, a significant rise in serum levels of the soluble form of ICAM-1 was observed in DR. These results suggest that peripheral blood immunoregulatory T lymphocytes are in a higher state of activation in DR as compared with early responders. The upregulation of ICAM-1 on these cells may be important in enhancing airway inflammation in patients with LAR.