A sensitive and reliable method was developed for the identification and quantitation of cannabinoids in blood. Samples were screened by fluorescence polarization immunoassay. Analysis was completed on a benchtop mass selective detector using selected ion monitoring. The limits of detection were 0.2 ng/mL for delta9-tetrahydrocannabinol (THC and 11-hydroxy-THC and 2 ng/mL for 11-nor-9-carboxy-THC. Extensive method validation is presented, including within-run variation, between-run variation, and results from external proficiency testing. Sample stability was studied over a 6-month period. Several derivatives and extraction techniques were evaluated to determine optimum performance. Data from a blind study of 217 samples were used to determine the predictive value of the screening procedure. The procedure is used routinely in the laboratory on samples from drivers issued a citation for impaired driving and also on postmortem blood from death investigations.