Monoclonal antibodies were produced and characterized in order to allow the monitoring of the urinary excretion of six modified nucleosides. The specificity of each antibody was determined and competitive solid-phase enzyme-linked immunoassays were designed, the sensitivity of which lay in the pmol range. Detection and quantitation of 5-methylcytidine (5-MeCyd), 4-acetylcytidine (4-AcCyd), 1-methylinosine (1-MeIno), 1-methyladenosine (1-MeAdo), 7-methylguanosine (7-MeGuo) and pseudouridine (psi-Urd) can be performed in small volumes (70 microliters) of untreated urine. Results can be obtained from as many as 20 different samples, for one molecule, within 3 h. With this technique, values observed for three commonly measured nucleosides in urine from healthy subjects (psi-Urd, 1-MeAdo and 1-MeIno) are in good agreement with those reported by other authors after analysis by high performance liquid chromatography. Results obtained in urine from cancer patients show significantly increased levels of the six haptens quantitated by this immunoassay.