An iodinated analog of the DNA-minor-groove-binding agent Hoechst 33342 has been synthesized and evaluated for DNA binding and tumor targeting. The bis-benzimidazole ring system of the title compound was constructed from the piperazinyl terminus via a Pinner-type cyclization followed by oxidative cyclization of the diamine Schiff base. To synthesize radioiodoHoechst 33342, (trimethylstannyl)Hoechst 33342 was prepared by the same strategy and subjected to mild radioiododestannylation in the presence of lactoperoxidase. After purification by HPLC, the radiochemical was separated in carrier-free form with > 85% radiochemical yield and > 99% chemical and radiochemical purity. Fluorescence spectrometric analysis of the binding of iodoHoechst 33342 to calf thymus DNA gave an equilibrium association constant (Ka) of 2.57 x 10(7) M-1 comparable to the Ka value of Hoechst 33342. Fluorescence microscopy of viable V79 cells demonstrated that the iodinated dye stained the nuclei with avidity similar to that of the noniodinated dye. The biodistribution of [125I]-iodoHoechst 33342 in LS174T tumor-bearing athymic mice 4 h postadministration showed a tumor uptake of 3-4% injected dose per gram (ID/g), tumor/blood ratio of 6-8, and tumor/ nontumor ratios above unity for most organs. A low thyroid uptake (approximately 2% ID/g) indicated that the radiochemical did not deiodinate and was stable in vivo.