The construction of a sheathless interface for capillary electrophoresis-electrospray ionization mass spectrometry (CE-ESI-MS), for operation with a Z-Spray(TM) source on a Micromass Quattro-LC(TM) triple quadrupole mass spectrometer is described. Designing the interface involved machining a probe compatible with the setup already in place on the mass spectrometer, i.e., MegaFlow-Z(TM) ESI. The probe was made of Lexan with the same dimensions as the ESI probe supplied with the instrument. The electrical connection at the electrospray end of the CE capillary was made possible by gold-coating (sheathless CE-ESI-MS). The probe design as well as the electrical and power supply requirements are described in detail. Experiments were performed using this interface, and CE separations of mixtures containing pmole and sub-pmole amounts of peptides were monitored by on-fine MS. For a standard peptide mixture (10(-4) M), separation efficiency was typically characterized by N > 10(4) theoretical plates with S/N > 400. Using the same experimental setup, ii was also possible to conduct on-line CE-ESI-tandem MS (MS/MS) experiments on the same peptide mixture, and to determine the sequence of the peptides. MS/MS scan functions for different precursor ions were used either alternately or sequentially and the results from both methods were compared. The possibility of peptide mass mapping was explored, and CE-ESI-MS results were obtained for the digestion products of equine myoglobin. Separation efficiencies and S/N values were similar to those obtained for standard peptides. A complete map of the digestion products was obtained.