We describe a homogeneous enzyme immunoassay for measurement of cannabinoid metabolites, as well as delta9-tetrahydrocannabinol (I) in urine. Malate dehydrogenase from pig heart mitochondria was labeled with a derivative of I. The compound used to calibrate the assay was the I metabolite, 11-nor-delta9-tetrahydrocannabinol-9-carboxylic acid (II). With 15 microgram of II per liter of urine as the cutoff concentration, the assay can detect 25 microgram of II per liter with greater than 95% confidence. A positive response was obtained for urine specimens assayed within 30 min after exposure to cannabinoids. However, the persistence of metabolites of I in urine indicates that assay of this fluid is useful as an indicator of cannabinoid use but not as an indicator of intoxication.