1. Human platelet thromboxane A2 receptor expression on the membrane surface is possibly dynamically regulated by changes either in the ligand concentration or in membrane fluidity. An increased thromboxane A2 production and a decreased membrane fluidity has been reported in diabetic patients. 2. In the present study the binding characteristics of platelet thromboxane A2 receptors have been investigated in nine diabetic patients (type I) and in 15 healthy control subjects by a radioligand-binding method using 9,11-dimethylmethane-11,12- methane -16-[3-125I-4- hydroxyphenyl]-13,14-dihydro-13-aza-15-tetranor-thromboxane A2 as the radiolabelled ligand. 3. The maximum concentration of binding sites was 163 (sd 35) fmol/108 platelets (n = 15) with 987 (sd 209) receptors/platelet in controls, whereas in diabetic patients the maximum concentration of binding sites was 74.2 (sd 28) fmol/108 (n = 9) with 447 (sd 172) receptors/platelet (P < 0.001). The dissociation constants were 18 (sd 4) nmol/l and 21 (sd 6) nmol/l (not significant) in control subjects and in diabetic patients, respectively. Glycated haemoglobin, which is reported to reduce membrane fluidity, was found to be negatively correlated (r = 0.60, P < 0.05) with thromboxane A2 receptor number in the diabetic patient group. On the contrary, a positive linear correlation between the equilibrium dissociation constant and glycated haemoglobin was found in diabetic patients (r = 0.75, P < 0.01).