As an assay system for mass screening of infants with neuroblastoma, a new method of determining urinary vanillylmandelic acid (VMA) and hemovanillic acid (HVA) levels--an enzyme immunoassay (EIA) method--was developed. By using two different monoclonal antibodies, one against VMA and the other against HVA, this system can assay 400 urine samples for both catecholamine metabolites in five hours. By measuring both VMA and HVA levels in 275 urine samples (62 from patients with neuroblastoma, 13 from patients with control tumors, 200 from healthy infants as controls) by the EIA method, as well as by high performance liquid chromatography (HPLC), we examined whether the EIA system is as accurate as the HPLC. There were significant correlations between values obtained by the two systems, both for VMA and for HVA, suggesting that the EIA method would be a more reliable and convenient system for mass screening of infantile neuroblastoma as compared with conventional qualitative tests with inevitable high false-positive rate.