Using the reverse transcription polymerase chain reaction (RT-PCR) we found that human neutrophils express mRNA for both A2A and A2B adenosine receptors, and using selective adenosine receptor agonists and antagonists we have characterized the type of adenosine receptor mediating inhibition of formyl-Met-Leu-Phe (fMLP)-induced oxidative burst. The order of potency of agonists was 5'-N-ethyl-carboxamidoadenosine (NECA) > 2-phenylaminoadenosine > 2-[p-(2-carbonyl-ethyl)-phenyl-ethylamino]-5'-N-ethyl-carboxamido adenosine (CGS 21680) > adenosine > N6-cyclopentyl-adenosine. This agrees with the agonist potency at human A2A receptors. The effect of adenosine was antagonized by 30 microM theophylline > caffeine = paraxanthine, i.e. concentrations close to those occurring in plasma after consumption of caffeine-containing beverages. The effect of NECA was unaltered by the A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine, but inhibited by the A2A receptor selective antagonists 4-amino-8-chloro-1-phenyl-[1,2,4]-triazolo[4,3-a]quinoxaline (CP 66,713), 1,3-dipropyl-8-(3,4-dimethoxystyryl) -7-methylxanthine (KF 17387) and 8-(3-chlorostyryl)caffeine as well as by the non-selective, non-xanthine antagonist 5-amino-9-chloro-2-(2-furyl)-[1,2,4]-triazolo-[1,5-c]quinazoline methane sulphonate (CGS 15943). The adenosine receptor mediated responses were antagonized by the protein kinase A blocker Rp-cyclic adenosine 3',5'-phosphorothioate (Rp-cAMP). In conclusion, the adenosine-induced inhibition of neutrophil activation is mediated by adenosine A2A receptors.