A novel dipyridodiazepinone, 6,11-dihydro-11-cyclopropyl-4-methyldipyrido[2,3-b:2',3'-e]- [1,4]diazepin-6-one (BI-RG-587), is a selective noncompetitive inhibitor of HIV-1 reverse transcriptase (RT-1). An azido photoaffinity analogue of BI-RG-587 was synthesized and found to irreversibly inhibit the enzyme upon UV irradiation. BI-RG-587 and close structural analogues competitively protected RT-1 from inactivation by the photoaffinity label. A thiobenzimidazolone (TIBO) derivative, a nonnucleoside inhibitor of RT-1, also protected the enzyme from photoinactivation, which suggests a common binding site for these compounds. Substrates dGTP, template-primer, and tRNA afforded no protection from enzyme inactivation. A tritiated photoaffinity probe was found to stoichiometrically and selectively label p66 such that 1 mol of probe inactivates 1 mol of RT-1.