In all of these specimens (representing 15 percent of all below-limit cases) either the police surgeon who carried out the clinical examination, or the police, strongly suspected that drugs may have been a contributory factor in driving impairment, considering the lower than expected alcohol concentration. Thirty-eight (18 percent) samples were found to contain significant drugs. Benzodiazepines were the most frequently encountered group of drugs (87 percent of all positive cases) with dizepam being the most frequently encountered (18 cases). The analytical procedures were radioimmunoassay, gas chromatography using nitrogen selective and electron capture detection along with high performance liquid chromatography (HPLC) using ultraviolet detection. Drugs and their metabolites were identified using a mixture of these techniques along with GC/MS where possible. The usefulness of HPLC coupled with a rapid-scanning diode-array spectophotometer is also demonstrated, the technique being particularly useful in the analysis of some of the more 'difficult' benzodiazepines (e.g., lorazepam, temazepam, nitrazepam) not directly amenable to gas chromatography without derivatization. (Publisher abstract)