The predominant circulating folate coenzyme in plasma/serum, 5-methyltetrahydrofolate (5-MTHF) was determined in human blood, serum and urine using a method based on the hyphenation of capillary ITP and zone electrophoresis. Measurements were done with a commercially available instrument for capillary isotachophoresis equipped with a column-switching system. The choice of electrolytes was limited by the instability of 5-MTHF and volatility of electrolytes for the potential coupling of the instrumentation with MS detector. To get an insight into the separability of individual sample components in an isotachophoretic analysis, we constructed zone existence diagrams for isotachophoretic electrolyte systems having a leading electrolyte composed of acetate and ammonium of pH 4.5 and 7.0, hydrocarbonate and ammonium, pH 7.8, chloride and ammonium, pH 5.6, and chloride and creatinine, pH 5.0, with hydroxide ion as the terminator. For isotachophoretic preseparation, the non-volatile leading electrolyte with good buffering capacity composed of 1 × 10(-2) M HCl and 2.5 × 10(-2) M creatinine, pH 5.0, and terminating electrolyte composed of 1 × 10(-2) M MES was selected as the most suitable. The optimum BGE for CZE analysis from the standpoint of analyte stability, separability and volatility for MS coupling was 1 × 10(-2) M acetate with 3.5 × 10(-2) M ammonium, pH 4.5. Using this combination of electrolytes, LODs reached with optical detection at 220 nm were 1.6 × 10(-7) M in human blood, 1.1 × 10(-7) M in human serum and 4.7 × 10(-6) M in human urine. Estimated content of 5-MTHF in blood and serum samples of women following oral daily administration of 0.8 mg of folic acid was 1.2 × 10(-5) and 5.8 × 10(-6) M, respectively.