A new bioassay system using an oligodeoxyribonucleotide (DNA) obtained by the in vitro selection method is described. The DNA aptamer which selectively binds to a target molecule, Reactive Green 19 (RG19), was labeled with fluorescein. The circular dichroism spectra revealed that the fluorescein labeling did not significantly affect the conformation of the DNA. The binding affinity toward RG19 of the labeled DNA was the same as that of the nonlabeled DNA. Using the labeled DNA and the RG19-immobilized gel, a semiquantitative assay of RG19 concentration was performed.