Six volunteers each smoked simultaneously, in a small unventilated room (volume 27 950 litre), a cannabis cigarette containing 17.1 mg delta 9-tetrahydrocannabinol (THC). A further four subjects - passive inhalers - remained in the room during smoking and afterwards for a total of 3 h. Blood and urine samples were taken from all ten subjects and analysed by radioimmunoassay for THC metabolites. The blood samples from the passive subjects taken up to 3 h after the start of exposure to cannabis smoke showed a complete absence of cannabinoids. In contrast, their urine samples taken up to 6 h after exposure showed significant concentrations of cannabinoid metabolites (less than or equal to 6.8 ng ml-1). These data, taken with the results of other workers, show passive inhalation of cannabis smoke to be possible. These results have important implications for forensic toxicologists who are frequently called upon to interpret cannabinoid levels in body fluids.