This paper presents an improved, comprehensive liquid chromatography-electrospray-mass spectrometry (LC-ES-MS) general unknown screening (GUS) procedure for drugs and toxic compounds and its comparison with conventional techniques in routine laboratory conditions. Chromatographic separation involved an X-TERRA MS C18, 3.5 microm (100 mm x 1 mm i.d.) column together with a 25-min long gradient of acetonitrile in pH 3, 2 mM ammonium formate delivered at a 50 microl/min flow rate. Two different in-source collision-induced dissociation voltages were alternated, both in the positive and in the negative ion modes. Reconstructed spectra were then obtained in both polarities by adding up spectra obtained with low and high energy, resulting in spectra presenting a sufficient number of specific fragment ions for unambiguous and fast identification of compounds. Two large mass spectral libraries of drugs and toxic compounds were built and an efficient automated signal processing, library searching and report editing algorithm developed. Using a common, efficient solid-phase extraction procedure, this LC-ES-MS technique was compared to GC-MS and HPLC-DAD GUS procedures for the identification of a priori unknown compounds in 51 serum samples consecutively sent to the laboratory for GUS. The present LC-MS method identified 75% of the compounds contained in these samples (versus 66% for GC-MS and 71% for HPLC-DAD), including 8% that the other two techniques failed to identify (versus 8% for GC-MS and 9.5% for HPLC-DAD). Therefore, it is complementary to GC-MS and/or HPLC-DAD and helps enlarge the range of drugs detected in clinical toxicology. It could be useful as well in forensic toxicology to confirm a positive result, as 38% of all the compounds were detected by the three techniques and 36% by two of them.