N-Acetyl-beta-D-glucosaminidase (NAG) activity has been measured in the serum and urine of primary and secondary diabetics and in primary diabetics with microangiopathy. NAG activity has also been measured in the tears of diabetics with ocular complications and diabetics with no ocular changes. Results have shown significantly higher levels of urinary NAG in diabetics with proteinuria (p less than 0.001) and proteinuria and retinopathy (p less than 0.001). There was no correlation between urinary NAG activity and serum creatinine (r = 0.28) or urinary NAG and the degree of proteinuria (r = 0.24). Increased urinary NAG levels were also observed in secondary diabetes associated with haemochromatosis and acromegaly. Significantly higher serum NAG levels were found in newly diagnosed diabetics (p less than 0.01) and significantly lower levels in chemical diabetics (p less than 0.01). Compared to non-diabetic controls tear NAG levels were significantly higher in the diabetic controls (p less than 0.01), in diabetics with retinopathy (p less than 0.01), and in diabetics with cataract formation (p less than 0.05). An assessment of this enzyme is made in relation to the development of diabetic microangiopathy.