One of the constituents of marijuana smoke, cannabinol, was found to photolytically decompose to one compound under nitrogen, but to three different compounds in the presence of air. The photolytic decomposition product formed under nitrogen was found to have intense fluorescent properties. Extracting saliva and irradiating the extract under nitrogen produced a detectable fluorometric change which was found to be adequate if more than 1 ng/ml of cannabinol is present in saliva.