A method to identify and accurately measure non-labelled Al-tetrahydrocannabinol (A1-THC) in blood of cannabis smokers has been developed. It consists of the following steps : To a 5 ml plasma sample is added deuterated A1-THC (A1-THC-d,) as internal standard. After extraction with light petroleum and evaporation, the A1-THC containing fraction is separated by chromatography on Sephadex LH-20 (1 x 40 cm) using light petroleum-chloroform-ethanol (10: 10: 1) as eluant. A fraction containing A1-THC is collected and subjected to mass fragmentography (LKB 9000; 3 % OV-l7/GasChrom Q; 230"). The mass spectrometer was adjusted to record the intensities of mle 299 and 314 of A1-THC and mle 301 and 316 of Al-THC-d,. The standard curve was made by plotting peak height m/e 299/m/e 301. Peak levels of 19-26 ng ml-l were reached within 10 min after smoking a cigarette containing 10 mg Al-THC.