The parameters which influence the determination of the major glycosylated haemoglobin fraction (HbA1c) with the thiobarbituric acid (TBA) method are described. Conditions for an optimal determination method are given. The correlation with the fast haemoglobin determination by column chromatography is greater than 0.9. The influence of storage on blood, washed erythrocytes and haemolysate in respect to HbA1c values obtained with the method described, was also investigated. Normal and pathological values from 78 patients correlate well with the fasting blood sugar levels (r greater than 0.9).