As is found by atomic absorption spectroscopy, the highly purified bovine tryptophanyl-tRNA synthetase contains up to 0.9 mol Zn2+/mol enzyme while some other bivalent metal ions are absent. The enzyme is inactivated either upon treatment with 1,10-phenanthroline (a zinc-chelating agent) or upon prolonged dialysis (which eliminates bound Zn2+ ions); addition of zinc reactivates the enzyme. Exposed histidine residue(s) and carboxylic group(s) of the enzyme are involved in the Zn2+ binding, as is shown using chemical modification. Circular dichroism spectra suggest that elimination of Zn2+ ions affects the tertiary rather than the secondary structure of the tryptophanyl-tRNA synthetase. The kinetics of inhibition with 1,10-phenanthroline toward ATP, tryptophan and tRNATrp indicates that removal of zinc prevents the ATP binding to the enzyme.