The need to precipitate bound insulin immediately after withdrawal of blood and the tendency to haemolysis, which reduces immunoassayable insulin, have prevented development of methods of self-collection of capillary blood for later free insulin measurement. We therefore investigated the use of the thiol-oxidizing agent, diamide, to prevent insulin loss with haemolysis and developed a self-collection procedure with capillary tubes pre-filled with diamide and polyethyleneglycol (PEG, for separation of free and bound insulin). Diamide (final concentration 5 mmol l-1) reduced serum insulin loss from 48 +/- 4 (+/- SE) to 11 +/- 4% (p less than 0.001) in maximally-haemolysed samples. The effect of diamide was concentration-dependent up to 5 mmol l-1. Diamide had no effect on the standard curve for radioimmunoassay of insulin. Levels of serum free insulin in self-collected capillary blood were significantly correlated with venous serum free insulin in 22 non-diabetic subjects (r = 0.92, p less than 0.001), 52 Type 1 diabetic patients (r = 0.86, p less than 0.001), and 18 Type 2 diabetic patients (r = 0.97, p less than 0.001). Mean capillary free insulin concentration was higher than in venous serum (22% in normal subjects, 64% in Type 1, and 23% in Type 2 diabetic patients). Storage at room temperature of capillary blood containing PEG/diamide for 72 h did not alter immunoassayable insulin concentrations.