Aldose reductase has been found in several tissues involved in the various complications of long-term diabetes. Since the loss of intramural pericytes is typical of early microangiopathy of the diabetic retina, the presence of aldose reductase and its activity were investigated in pericytes cultured from capillaries of human retinas. Verification that contaminating cells had been removed was made on the basis of the pericytes' distinctive appearance in culture, inability to internalize acetylated-low-density lipoprotein, and immunoreactivity for muscle actin. Using pure cultures of human pericytes, the presence of aldose reductase was demonstrated immunohistochemically with antibodies directed against human placental aldose reductase, and aldose reductase activity was shown biochemically by monitoring the accumulating of xylitol in cells incubated with xylose. Xylitol accumulation was inhibited by the inclusion of an aldose reductase inhibitor in the xylose-containing medium.