Anti-melatonin monoclonal antibodies (MAbs) of high titer were prepared by coupling melatonin to bovine serum albumin with formaldehyde and by immunizing BALB/c mice with multifocal intradermal injections and by fusing high titer antibody producing spleen cells with myeloma cells of SP2/0 origin. Five MAbs were selected for further characterization as classes and subclasses. After four successive limiting dilutions, antibodies were produced by these five clones with high affinities ranging from 10(9) to 10(11)/m. These clones were found to be of the immunoglobulin Ig G1 and IgG(2b) subclass with kappa light chain. A systematic study of cross-reactions with seven compounds (indole, aromatic and imidazole derivatives) showed that the antibody had a high specificity for melatonin, low reactivity with 6-hydroxymelatonin and N-acetyl-5-hydroxytryptamine, and no detectable reactivity with tryptamine, l-tryptophan, 5-methoxytryptamine and N-acetyl-L-tryptophan. The roles of the indole nucleus and the side chain in the determination of the antigenic properties of the molecule are discussed. One of the MAbs, 4C9D7, was used to establish a competitive enzyme-linked immunosorbent assay for the detection of melatonin in supernatant.