Plasma glutathione S-transferase basic isoenzyme (GST B1) concentrations have been measured by specific radioimmunoassay in Type 1 diabetic patients and in normal subjects, before and after controlled insulin-induced hypoglycaemia, and in a further group of Type 1 diabetic patients in hypoglycaemic coma. The activities of alanine aminotransferase (ALT), aspartate amino-transferase (AST), and gamma-glutamyl transferase (gamma GT) were also measured. GST B1 concentrations were significantly increased 3 h after controlled insulin-induced hypoglycaemia, both in the diabetic patients (p less than 0.02) and in the normal group (p less than 0.05), but the magnitude of the rise did not differ between these two groups. Four of the 9 patients presenting in hypoglycaemic coma had a GST B1 concentration above the reference range. ALT, AST, and gamma GT activities did not rise following hypoglycaemia in any of the groups.