Adenosine mediates a wide range of physiological functions by activation of at least four cell surface receptors named A1, A2A, A2B and A3. Each receptor subtype has been cloned and shows unique ligand binding properties and a distinct pattern of tissue expression (Fredholm et al., 1994; Linden, 1994; Alexander & Peters, 1997; Ralevic & Burnstock, 1998). Adenosine is a highly active biologic compound with a variety of effects on numerous tissues, including heart muscle, coronary arteries, smooth muscle cells, platelets and cells involved in immune and inflammatory reactions. Through interaction with A2A receptors, adenosine is involved in platelet antiaggregatory effects, neutrophil antiinflammatory responses as well as in modulation of immune cell function (Salmon & Cronstein, 1990; Sullivan et al., 1990; Hoskin et al., 1994; MacKenzie et al., 1994). The presence of A2A receptors on all these different cell types ie monocytes, lymphocytes, neutrophils, basophils, mast cells should not be surprising because the cells involved in the immune and inflammatory responses arise from a common stem cell source in the bone marrow. Studies on the effects of adenosine and adenosine analogues on blood cells are important due to the proposed role of adenosine (1) in the pathogenesis of diseases such as severe combined immunodeficiency (ADA SCID)(Giblett et al., 1972; Huang et al., 1997),(2) in the regulation of normal immune processes (Bouma et al., 1997; Apasov et al., 1997),(3) as an endogenous antiaggregatory and anti-inflammatory agent (Cronstein, 1995; Sullivan & Linden, 1998) and (4) in the use of adenosine analogues as pharmacologic agents (Jacobson et al., 1992; Olah & Stiles, 1995; Poulsen & Quinn, 1998). On this basis the present review summarizes the pharmacological, biochemical and functional data of A2A receptors expressed in human platelets, lymphocytes and neutrophils of peripheral blood and describes the signal transmission mechanisms and the functional responses responsible for adenosine's biological effects.