Cannabis use is confirmed through the analysis of blood and urine using combined high-performance liquid chromatography/radioimmunoassay to detect delta 9-THC-11-oic acid and its O-ester glucuronide (the major delta 9-THC metabolites). The method is sensitive and specific and will allow the complete analysis of at least six samples per day. It has been successfully applied to the analysis of samples submitted for forensic analysis and, in combination with a previously described radioimmunoassay, offers a useful approach to routine toxicological analysis of cannabinoid metabolites.