Plasma beta-thromboglobulin was measured, using the Edinburgh radioimmunoassay technique and anticoagulant mixture (containing prostaglandin E1) in 61 normal subjects, 67 diabetics with and 54 diabetics without microangiopathic complications. Plasma beta-thromboglobulin was significantly higher in the diabetic patients (p < 0.01) but there was no significant difference between the two diabetic groups. Twenty-six normal subjects, 27 diabetics with and 39 diabetics without complications were studied further by measuring beta-thromboglobulin in four different ways using two different radioimmunoassay techniques and two anticoagulant mixtures (with and without prostaglandin E1). The Edinburgh assay gave a value 1.97 times that obtained with the Amersham assay, and the Edinburgh anticoagulant a value 0.78 times that with the Amersham anticoagulant. Beta-thromboglobulin concentration in the meniscus layer was approximately twice that in the middle layer. The lower beta-thromboglobulin values obtained with the Edinburgh anticoagulant may result from a different sampling technique or from prevention of in vitro beta-thromboglobulin release, after venepuncture, by prostaglandin E1. Abnormal platelet behaviour in diabetes was confirmed although its role in the pathogenesis of microangiopathic complications remains unclear.