Formaldehyde conjugate of serotonin. Without paraformaldehyde treatment, it cross-reacts in liquid phase with dopamine and tryptamine in haemagglutination tests. The antibody reacts with serotonin in neurons and recognises serotonergic sites in fixed tissue sections. It does not react with catecholamine-containing neurons in formaldehyde fixed preparations. By immunocytochemistry no cross-reactivity with dopamine and noradrenaline (6). By red cell agglutination, no cross-reactivity with 5-hydroxyindolacetic acid, GABA, noradrenaline, 5-hydroxytryptophan, carnosine, melatonin