Aldose reductase (AR) protein was measured in peripheral mononuclear cells (PMCs) from 55 patients with type 2 diabetes by a two-site ELISA using anti-human AR monoclonal antibody. AR levels did not correlate with age, duration of diabetes, and HbAlc. Furthermore, no significant differences were found in AR levels between the patients and healthy subjects. Thirty seven patients had at least one of diabetic microangiopathy; retinopathy, neuropathy, or nephropathy. AR levels were significantly higher in the patients with microangiopathy than in those without it (52.3 +/- 15.7 vs. 43.0 +/- 15.2 ng/10(6) cells, P < 0.05). The patients with neuropathy had significantly higher AR levels than those without neuropathy (53.7 +/- 15.8 vs. 42.7 +/- 14.3 ng/l0(6) cells, P < 0.05). The same result applied to the patients with retinopathy (54.5 + 15.4 vs. 44.6 +/- 15.3 ng/10(6) cells, P < 0.05). The AR levels in the patients with nephropathy tended to give a higher value than those without it. However, there were no significant differences between the two (53.9 +/- 3.6 vs. 46.4 +/- 2.6 ng/10(6) cells, NS). These results indicate that AR levels in PMCs from type 2 diabetic patients are associated with the presence of microangiopathy. The measurement of AR proteins in PMCs with this ELISA system is a useful tool for the clinical study of diabetic complications, and would increase our understanding of the pathogenesis of the disease.