A survey on the quality of drugs-of-abuse testing in European laboratories was performed in 1993 (Part I) and 1994 (Part II). A total of 195 laboratories participated in Part I and 228 in Part II. There were 154 repeater laboratories. In each part of the survey, six urine samples were sent for analysis under routine conditions. A set of reference materials, including deuterated drug-standard solutions, was provided in Part II of the survey for optimization of analytical procedures. Screening for groups of substances was mainly performed by immunological techniques. Rates of false- positive (FP) results were 0 and 0.5%, respectively, in Parts I and II, and rates of false-negative (FN) results were 7.7% in Part I and 3.4% in Part II. Identification and quantitation of specific substances was mainly carried out by chromatographic methods, particularly gas chromatography coupled to mass spectrometry. Application of chromatographic methods showed a lack of sensitivity (14.9% and 16.7% FN in Parts I and II, respectively) and a lack of specificity (0.8% FP in Part I versus 1.1% in Part II). Repeater laboratories improved their results (17.4% FN in Part I versus 14.2% Part II). The improvement of repeater laboratories emphasizes the need of external quality control programs. The experience is being used as a basis for a recommendation on drug-testing guidelines by a group of European toxicologists.