New carbamazepine hapten analogues are described comprising: (A) an active ester group; (B) a carbamazepine nucleus; and (C) a linking chain (i) linking the carboxamide group of the carbamazepine nucleus to the active ester group, said linking chain having about 4 to about 40 atoms consisting of: (1) alkylene groups; and (2) 5 to 7 membered heterocyclic ring groups, each group being joined into the linking group through chemical groups selected from (a) esters, (b) amides, (c) hetero atoms selected from --0--, --S--, and --NR--; wherein R represents hydrogen or C.sub.1 to C.sub.6 alkyl; and (d) carbonyl groups. The carbamazepine-active ester analogues are useful in preparing labeled carbamazepines. The labeled carbamazepines are useful in immunoassay elements and processes for the detection of carbamazepine drugs, for example, in body fluids.
