The rates of glycosylation of lens proteins were determined in extracts of human 'diabetic' and 'senile' cataractous lenses by a method employing thiobarbituric acid. Incubation of soluble lens proteins (6,500 X g supernatant of homogenates) in vitro with various concentrations of D-glucose in sodium phosphate buffer (50 mmol/l, pH 7.2) resulted in a gradual glycosylation which was time and concentration dependent. Glycosylated proteins from the cataractous lenses of 21 senile and 12 diabetic subjects afforded 0.72 +/- 0.22 and 1.84 +/- 0.44 nmol 5-hydroxymethylfurfural/mg protein (mean +/- SD), respectively. The value is significantly higher in the diabetic than in the senile group (p less than 0.001), although the mean age of the diabetic patients (67 years) was significantly younger than that of senile subjects (75 years; p less than 0.01). These results indicate that human lens proteins can be glycosylated both in vitro and in vivo, and that hyperglycaemia can accelerate the non-enzymatic glycosylation of lens proteins in diabetic patients.