Human neutrophils have been reported to possess both alpha 2- and beta 2-adrenoceptors. While activation of beta 2-adrenoceptors is known to inhibit N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP)-induced superoxide anion (O2-) production, the functional role of alpha 2-adrenoceptors is not known. We studied the effects of a range of structurally unrelated alpha 2-adrenoceptor agonists on fMLP-induced O2- production and UTP-induced increases in cytosolic free calcium concentration ([Ca2+]i) in human neutrophils. No effect of alpha 2-adrenoceptor agonists was seen on either fMLP-induced O2- production or UTP-induced increases in [Ca2+]i. alpha 2-Adrenoceptor agonists by themselves had no effect on either O2- production or [Ca2+]i. We then studied a model for neutrophils, differentiated HL-60 cells and human erythroleukaemia (HEL) cells, a cell line known to possess alpha 2-adrenoceptors. While the alpha 2-adrenoceptor agonists 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304) and 5-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]azepin- dihydrochloride increased the [Ca2+]i in HEL cells, they had no effect by themselves on either [Ca2+]i or UTP-induced increases in [Ca2+]i in differentiated HL-60 cells. Activation of high-affinity GTPase by UK 14304 was seen in membranes from HEL cells but not in membranes from differentiated HL-60 cells. Similarly, a selective alpha 2-adrenoceptor antagonist, [3H]2-(2-methoxy-1,4-benzodioxan-2yl)-2 imidazoline, bound specifically and saturably to membranes from HEL cells, but not to membranes from HL-60 promyelocytes or differentiated HL-60 cells. Taken together, these data suggest that neither HL-60 promyelocytes nor differentiated HL-60 cells possess alpha 2-adrenoceptors, and that the lack of functional responses to alpha 2-adrenoceptor agonists in human neutrophils is due to the absence of alpha 2-adrenoceptors.