MAb against delta 1-THCA was produced by fusing hybridoma with splenocytes immunized with delta 1-THCA-BSA conjugate and hypoxanthine, aminopterine, thymidine-sensitive mouse myeloma cell line, P3-X63-Ag8-653. The cross-reaction of anti-delta 1-THCA antibody against other cannabinoids was very wide, thus many cannabinoids and a spiro-compound were reactive suggesting that 2'-hydroxyl, 6'-hydroxyl or 6'-O-alkyl, 4'-alkylbenzene ring moiety is necessary for its reactivity. It became evident that this ELISA was able to be applied to the biotransformation experiments of cannabinoids in plant tissue culture system. The metabolites of delta 6-THC such as two major metabolites, 7-oxo-delta 6-THC and 7-hydroxyl-delta 6-THC were also detectable by this ELISA.