Following initial screening using either an established radioimmunoassay (RIA) or a novel EIA procedure, a quantitative estimate was established using a high performance liquid chromatography (HPLC) technique following solid phase extraction. Final confirmation and quantitation, without derivatization, was established using HPLC in combination with electrospray ionization mass spectrometry (MS) and using methysergide as an internal standard. The detection limit of LSD in urine was 0.5 ng/ml, and a blind trial confirmed the validity of the results. The choice of internal standard is discussed, with consideration paid to the photosensitivity of LSD solutions. A study of potential interferants in the HPLC-MS confirmation of LSD indicates that, for the wide range of compounds studied, none interfere with this confirmation technique. A comparison is shown between solid phase and immunoaffinity extraction/cleanup procedures and between RIA and EIA screening procedures. The authors conclude that HPLC-MS provides unequivocal identification of underivatized LSD in urine, with quantification down to a detection limit of 0.5 ng/ml. 12 references, 4 tables, and 10 figures