D-(-)-2-(6'-hydroxy-7'-[(123)I]iodobenzothiazolyl)-delta(2)-thiazoline-4-caroxylic acid (7'-[(123)I]iodo-D-luciferin) was synthesized as a novel reporter probe for in vivo studies of firefly luciferase gene expression. 7'-Iodo-D-luciferin, a nonradioactive standard, was synthesized and showed the binding property (K(M)=4.28 microM) similar to that of D-luciferin (2.53 microM) for firefly luciferase in luminescence assay.