A series of 5-deazaflavo-10,11-quinones having o-quinone structure in the molecule were synthesized. The cytotoxicity of 5-deazaflavo-6,9-quinones (p-quinone derivatives) and 5-deazaflavo-10,11 quinones (o-quinones) was evaluated in vitro against L1210 and KB cells. Some of the synthesized compounds exhibited cytotoxic activity comparable to that of mytomycin C.