The potential for cross-reactivity of an antiserum used to detect 11-nor-delta 9-tetrahydrocannabinol-9-carboxylic acid in a urine sample by a radioimmunoassay procedure has been examined. The degrees of cross-reactivity of 41 cannabinoids and non-cannabinoid phenolic constituents of Cannabis have been determined. In general, only (delta 8- or delta 9-THC type) cannabinoids of the dibenzopyran type structure were found to appreciably cross-react with the antiserum. Cross-reactivity was not observed with any non-cannabinoid constituent.