Modulation of neutrophil responses by adenosine may have an important role in limiting tissue injury during inflammation or ischemia-reperfusion. Mac-1 (CD11/CD18), a member of the leukocyte integrin family, participates in neutrophil adhesion to endothelium, in transendothelial migration, and in phagocytosis. Using monoclonal antibodies and flow cytometry, we investigated the effect of adenosine on the increase in plasma membrane expression of Mac-1 which occurs following stimulation of neutrophils with the chemotactic peptide N-formylmethionylleucylphenylalanine (FMLP). Adenosine and 5'N-ethylcarboxamido-adenosine, a potent A2 agonist, each produced a dose-dependent inhibition of as much as 50% of the increase in Mac-1 expression on neutrophils stimulated with FMLP, with an IC50 of approximately 1 nM. The effect of adenosine was blocked by 8-p-sulfophenyltheophylline, an adenosine-receptor antagonist, N6-cyclopentyl-adenosine, an A1-selective agonist, had no effect on FMLP-stimulated Mac-1 expression in the concentration range expected for its action on neutrophil adenosine receptors of the A1 type. We also found that dibutyryl cyclic adenosine monophosphate inhibited the upregulation of Mac-1, and that the effect of adenosine on Mac-1 expression was not reversed by colchicine or vinblastine. We conclude that adenosine acts via A2 receptors to inhibit the upregulation of Mac-1 expression of FMLP-stimulated neutrophils, and that A1 receptors are not involved. This effect of adenosine may help to limit Mac-1-dependent neutrophil exudation at sites of inflammation or ischemia-reperfusion.