Antibody specific for a 5-HTOL compound. A glucuronide with 5-hydroxytryptophol (5-HTOL) characterized in that it comprises the structure of the .beta.-glucuronide between the 5-hydroxy group of 5-HTOL and D-glucopyranosiduronic acid. An immunoassay and a diagnostic method utilizing the immunoassay wherein a 5-hydroxytryptophol (5-HTOL) compound is determined by the use of an antibody specific for a 5-HTOL compound is used.