Various thioxo-quinazolino[3,4-a]quinazolinones were prepared and evaluated for their cytotoxicity in MOLT-4 (lymphoblastic leukemia) and MCF-7 (breast adenocarcinoma) cell lines. Synthesis of the target compounds was started from isatoic anhydride. Successive reaction of isatoic anhydride with benzylamine and 2-nitrobenzaldehyde, reduction of the nitro group, and reaction with CS2 gave 12-benzyl-6-thioxo-6,7,11b,12-tetrahydro-13H-quinazolino[3,4-a]quinazolin-13-one. The latter compound reacted with various 2-chloro-N-substituted acetamides to afford the corresponding fused quinazolinone derivatives.