In the search for an alternative to chemical halogenation reactions using the free halogens, a novel type of halogenating enzymes was detected. In contrast to haloperoxidases, these NADH-dependent halogenases are specific. Tryptophan halogenase which catalyses the regioselective chlorination of tryptophan to 7-chlorotryptophan can also chlorinate tryptamine, tryptophol, indole-3-acetonitrile, and 3-methylindole. However, indole-3-acetonitrile is not chlorinated in the 7-position, but in positions two and three of the indole ring. Chlorination in the 3-position is obviously stereospecific. In addition to tryptophan and indole derivatives, aminophenylpyrrole is also accepted as a substrate for regioselective chlorination. Since the new NADH-dependent halogenases have a fairly broad substrate specificity and catalyse regioselective chlorination reactions they could be a good alternative to chemical halogenation.