Proteins purified by affinity chromatography on Blue-Sepharose CL-6B from serum of 10 normal controls and 19 Type I diabetic patients were studied by means of combined ultrathin isoelectric focusing and photochemical silver stain. While only a single band of protein (characterized as albumin by crossed immunoelectrophoresis) with a pI of 4.7 was found in serum of normal subjects, 10 out of the diabetic group showed some bands of proteins with a pI greater than 4.7 and a single one with a pI less than 4.7. Concanavalin A-Sepharose removed all these bands with altered pIs which were further characterized as albumin by fused rocket immunoelectrophoresis of the eluates from Concanavalin A-Sepharose, direct immunofixation after isoelectric focusing of proteins with high purified anti-albumin antibodies, SDS-polyacrylamide gradient pore electrophoresis, aminoacidic analysis. The gas-chromatographic analysis of carbohydrates released from both the albumin bound to Concanavalin A-Sepharose and that not bound, revealed in addition to two unidentified peaks, the presence of glucose, galactose and mannose whose contents were greatly increased in albumin with affinity for the lectin. Serum glycosyl albumin concentration was not statistically different in serum of diabetic patients displaying cationic glycosyl albumin in comparison to patients without these proteins (0.2261 +/- 0.0186 versus 0.1874 +/- 0.015 nmole HMF/nmole albumin), whereas the first group showed statistically higher urinary excretion rates of albumin (28.6 +/- 1.2 micrograms/min versus 4.6 +/- 0.2 micrograms/min).(ABSTRACT TRUNCATED AT 250 WORDS)