A new cell-based galactose assay system, which is comprised of two bioluminescent Escherichia coli strains immobilized within an agarose gel arrayed on a well plate, has been developed. For this purpose, a galT knockout strain [galT(-) cell] of E. coli was genetically constructed so that cell growth is not promoted by galactose but rather by glucose present in a sample. Another E. coli W strain (normal cell), which grows normally in the presence of either glucose or galactose, was employed. A luminescent reporter gene, which produces luminescence as cells grow, was inserted into both of the E. coli strains, so that cell growth could be monitored in a facile manner. The two strains were separately grown for 4 h on gel arrays to which test samples were individually supplied. The relative luminescence unit (RLU) values caused by cell growth were determined for each array, one of which is resulted by glucose only and the other of which is resulted by both glucose and galactose present in the sample. By employing this protocol, galactose concentrations present in the test sample are reflected in the differences between the RLU values for each array. The practical utility of the new assay system was demonstrated by its use in determining galactose levels in clinical blood spot specimens coming from newborn babies. Because it can be employed to diagnosis of galactosemia in newborn babies in a more rapid, convenient, and cost-effective manner, this cell-based solid-phase galactose assay system should become a powerful alternative to conventional methods, which require labor-intensive and time-consuming procedures and/or complicated and expensive equipment.