Methodology for the detection of delta 9-tetrahydrocannabinol (THC) in blood is reported. THC is extracted from blood with chloroform, and extracts are purified using paper chromatography. Identification of delta 9-tetrahydrocannabinol, cannabidiol, and cannabinol is achieved by the combined use of computerized gas chromatography-mass spectrometry. The mass spectrometer is operated in the multiple ion monitoring mode, monitoring ions of m/e 231, 238, 243, 271, 295, 299, 310, and 314. The procedure was developed for forensic purposes. The detection limit of THC is 0.5 ng/mL of blood. Data of 142 cases are presented.