We have examined the effect of glipizide, a hypoglycemic sulfonylurea, upon transglutaminase activity in human red blood cells. In a first series of experiments the in vitro effect of the drug was assessed. The results obtained showed that glipizide inhibits transglutaminase activity in human red blood cells. In a second approach, glipizide was administered orally to six type 2 diabetic patients during 3 months, in order to evaluate the long-term effect upon transglutaminase activity. Again, glipizide induced a significant decrease in the enzyme activity in blood red cells (P less than 0.01). We suggest that treatment of type 2 diabetes mellitus with hypoglycemic sulfonylureas could improve insulin effects by inhibiting cellular transglutaminase activity.