The invention describes the preparation and use of fluorescent stains for nucleic acids derived from neutral unsymmetrical cyanine dyes comprising a substituted benzazolium ring system linked to a methine bridge to a pyridine or quinoline ring system. The fluorescence characteristics of the dyes when complexed with nucleic acids give the dyes utility for the detection of oligonucleotides and nucleic acids in cells, gels and solutions. The dyes have particular utility in the staining of reticulocytes.