ISHIWATA, S., ITOH, K., YAMAGUCHI, T., ISHIDA, N. and MIZUGAKI, M. Comparison of Serum and Urinary Levels of Modified Nucleoside, 1- Methyladenosine, in Cancer Patients Using a Monoclonal Antibody-Based Inhibition ELISA. Tohoku J. Exp. Med., 1995, 176(1), 61-68-To evaluate the clinical usefulness of serum 1-methyladenosine, several modifications have been made in our previously established inhibition ELISA system. Horseradish peroxidase (HRP) labeled anti-mouse IgG and 3, 3', 5, 5'-tetramethylbenzidine (TMBZ) were used as a secondary antibody and a substrate, respectively. The second blocking was done just before the addition of the secondary antibody. The standard curve of the modified ELISA system showed good linearity between 1 and 1, 000ng/ml, and the detection limit was 50pg/well. Using the ultrafiltrated- serum samples, serum 1-methyladenosine levels in healthy individuals and cancer patients were determined. The mean level of 1-methyladenosine in 31 healthy individuals was 28.3±7.9ng/ml, and cut off value was set at 44.1ng/ml (Mean+ 2SD). In cancer patients, elevated levels of serum 1-methyladenosine above the cut off value were detected in 4 out of 25 cases tested, though 11 cases had elevated urinary 1-methyladenosine levels above the cut off value (3.23nmol/μmol reatinine). Since 1-methyladenosine has no interaction with serum proteins and its molecular weight is quite low, it might be rapidly excreted into the urine.