The effect of the O6-alkylguanine-DNA alkyltransferase (AGT) inhibitor, O6-benzylguanine (O6-BG), on the anti-tumour activity of 8-carbamoyl-3-methylimidazo [5,1-d]-1,2,3,5-tetrazine-4(3H)-one (temozolomide) or 1,3-bis(2-chloroethyl)-nitrosourea (BCNU) was evaluated in athymic mice bearing subcutaneous (s.c.)human glioma (U87MG) xenografts. Here, AGT is linked to central nervous system cancer.