Antithrombin III activity was determined by three techniques in 58 patients with maturity onset diabetes mellitus: a spectrophotometric assay, using the chromogenic substrate Chromozym TH; evaluation of antifactor Xa activity and an immunochemical technique. Although the biological antithrombin III activity was clearly reduced in individual subjects, there was only a slight nonsignificant reduction in the mean antithrombin III activity values as compared to normal subjects. No difference was observed between diabetics receiving insulin and subjects taking oral hypoglycemic agents or between diabetics without vascular complications and diabetics with vascular lesions.