Plasma and platelet taunine concentrations were assayed in 39 patients with insulin-dependent diabetes mellitus (IDDM) and in 34 control subjects matched for age, sex, and both total and protein-derived daily energy intake. Platelet aggregation induced by arachidonic acid in vitro at baseline and after oral taunine supplementation (1.5 g/d) for 90 dwas also studied. Plasma and platelet taunine concentrations (x ± SEM) were lower in diabetic patients (65.6 ± 3.1 p.mol/L, or 0.66 ± 0.07 mol/g protein) than in control subjects (93.3 ± 6.3 p.mol/L, or 0.99 ± 0.16 mol/g protein, P < 0.01). After oral supplementation, both plasma and platelet taunine concentrations increased significantly in the diabetic patients, reaching the mean values of healthy control subjects. The effective dose (x ± SEM) of arachidonic acid required for platelets to aggregate was significantly lower in diabetic patients than in control subjects (0.44 ± 0.07 mmol compared with 0.77 ± 0.02 mmol, P < 0.001, whereas after taunine supplementation it equaled the mean value for healthy control subjects (0.72 ± 0.04 mmol). In in vitro experiments, taunine reduced platelet aggregation in diabetic patients in a dose-dependent manner, whereas 10 mmol taunine/L did not modify aggregation in healthy subjects.