We developed a new interface for the on-line coupling of a fiber for solid-phase microextraction (SPME) with a CE system with an electrospray interface for MS detection. The on-line interface was a modification of a split-flow interface typically used for coupling flow systems to CE. The modification consists of an additional channel closed with a septum that permits the introduction of the fiber in the interface, right at the inlet region of the separation capillary. The interface was designed to be used with commercial fibers. Although commercial fibers can be used directly, we recommend cutting the stationary phase and using fibers with a 2-mm stationary phase. The system was tested for the analysis of tetracycline antibiotic residues, viz., tetracycline, oxytetracycline and doxycycline in soils. The overall accuracy was assessed by regression analysis of spiked samples, showing successful results at a 95% confidence level. The reproducibility, expressed as RSD, was 5.3% and the LOD of tetracycline antibiotics in soil ranged from 2.9 to 3.2 mu g/kg.