Platelet alpha adrenoceptor status was examined using the radioligands 3H-yohimbine (3H-YOH) and 3H-dihydroergocryptine (3H-DHE) in 14 patients with myeloproliferative disorder (MPD) and 10 normal controls. Platelets from normal controls and MPD patients sensitive to adrenaline induced aggregation exhibited approximately 50% more binding sites identified by 3H-DHE than 3H-YOH, whereas MPD platelets insensitive to adrenaline showed selective loss of these 'extra' 3H-DHE sites. In functional studies after 30 minutes preincubation with the unlabelled antagonists, DHE was more potent than YOH at inhibiting adrenaline induced aggregation in normal platelets. In addition, the affinity constant for DHE was virtually identical in binding and functional experiments, whereas for YOH the affinity constant for binding was approximately 10 fold more potent than that for aggregation. These results suggest that the alpha adrenoceptor binding site on human platelets labelled by 3H-DHE may be of more functional relevance than that labelled by 3H-YOH alone.