One of the major disadvantages of capillary electrophoresis (CE) is its limited loadability. Therefore, the on-line coupling of isotachophoresis (ITP) and CE was studied with regard to its potential for the improvement of the minimum concentration that can be measured by CE. Based on the concentrating and separating power of ITP, detection limits could be lowered by at least two orders of magnitude. Especially for biological samples containing proteins, it appeared that in non-treated capillaries the electromigration characteristics are hardly influenced when isotachophoretic pretreatment is applied. The potential of ITP-CE coupling is illustrated by the analysis of o-phthaldialdehyde and fluorescein isothiocyanate derivatives of a number of amino acids.