Capillary isotachophoresis is a useful method for the analysis of many ion components of various sizes of molecules. Analysis of human serum protein was performed with the Shimadzu capillary isotachophoresis IP-3A. The leading electrolyte system consisted of 5.0 mM hydrochloric acid with 10 mM ammediol as the counter ion and the viscous agent (0.2% HPMC 4000) was added to the leading electrolyte. The terminating electrolyte was 10 mM 6-amino caproic acid with 10 mM ammediol as buffer, corrected to pH 10.8 by the addition of barium hydroxide. IP-3A can be fitted three types of capillary tube of 0.5 mm I.D., 0.3 mm I.D. and 0.2 mm I.D. The human serum was mixed with spacer solution (2% ampholine and amino acid mixture) at a volume ratio 1:3.1.0 microliter of preparation mixture was injected and detected by UV-signal at 280 nm with capillary tube of 0.5 mm I.D. 1.0 microliter of 10 fold preparation mixture was analyzed with capillary tube of 0.2 mm and 0.3 mm I.D. The analysis with the capillary tube of 0.2 mm I.D. or 0.3 mm I.D. is more sensitive than that of 0.5 mm I.D., but the system with smaller I.D. tubes had a tendency to get clogged. Human sera of 61 persons were respectively mixed with spacer solution at a volume ratio 1:3 and each 1.0 microliter of preparation mixture was injected and detected by UV-signal at 280 nm with capillary tube of 0.5 mm I.D. The peak height ratios (albumin minor peak height/albumin main peak height) were classified into 3 types.(ABSTRACT TRUNCATED AT 250 WORDS)