Glycosylated hemoglobin (GHb) was measured by a chromatographic method using HPLC (HbA1c) as well as a colorimetric method using thiobarbituric acid (TBA) on samples collected from 40 diabetic subjects. GHb measured by HPLC correlated well with the colorimetric method (r = 0.7). Freezing the samples caused an immediate increase of the HbA1a+b but not the HbA1c results. 4 weeks storage of the red cells at room temperature, 4 degrees C or -20 degrees C altered the results of GHb measured by HPLC but not the TBA reaction. Half of the total ketoamine linked hexoses detected by the TBA reaction was found in the HbA0 fraction as measured by ion exchange chromatography. The colorimetric method (r = 0.53) had better correlation than the HPLC method (r = 0.27) with mean blood glucose values during the 4 weeks preceding sample collection. GHb measured by the TBA reaction more accurately represents the ketoamine linked sugar in hemoglobin, is less subject to handling artifacts and more accurately reflects the blood glucose values during the preceding 4 weeks.