The DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT, alkyltransferase) is an important suicide enzyme involved in defense against O6-alkylating endogenous metabolites and environmental carcinogens. It also plays a pivotal role in primary and acquired resistance of tumors to alkylating anticancer drugs targeting the O6-position of guanine (i.e., methylating and chloroethylating agents). MGMT can thus be considered a crucial biomarker for individual susceptibility to alkylating carcinogens and tumor drug resistance. This implies a need for a fast and convenient method for detn. of MGMT. Routinely, MGMT is being quantified by radioactive assays which are relatively laborious. Here we report a nonradioactive MGMT ELISA for quantification of MGMT in cell and tissue homogenates. We compared the MGMT-ELISA with the std. radioactive assay and found it to be as sensitive but less time consuming. Therefore, it represents an alternative for the quantification of MGMT in cell and tissue homogenates. We applied the assay for detg. MGMT in normal and tumor tissue of testes. In both normal and tumor tissue MGMT was quite variable, ranging from zero to 1300 fmol/mg protein. In various tumor samples MGMT was lower than MGMT in the normal tissue from the same patient or was even not detectable. The MGMT-ELISA might become a useful tool for MGMT detn. in clin. routine and health control. [on SciFinder (R)]