Gas chromatography-mass spectrometry (GC-MS) and immunological methods, including the Syva enzyme multiplied immunoassay technique, the Abbott fluorescence polarization immunoassay, and the Roche OnLine immunoassay, were compared for the determination of 11-nor-delta 9-tetrahydrocannabinol-9- carboxylic acid (THCCOOH). The results of all three immunoassays were not in accordance with the GC-MS results in three cases of a 72-specimen panel. Only one false negative was observed using the OnLine immunoassay. The immunological methods compared favorably and are acceptable for detecting the presence of cannabis metabolites in urine. These results support the concept that all immunoassays for cannabinoids should be considered as screening procedures. No concentration correlation between GC-MS and the immunoassays could be established because of the different cross-reactivities of the metabolites.