Toluidine blue-O is a Nissl bright-field counterstain for lipophilic fluorescent tracers Di-ASP, DiI and DiO.

The stain toluidine blue-O (tol blue), applied to sections of neural tissue, is shown to be compatible with the vivid fluorescent lipophilic neural tracers 4-(4-dihexadecylaminostyryl)-N-methylpyridinium iodide (Di-ASP), 3,3'-dioctadecyloxacarbocyanine perchlorate (DiO) and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI). As with other Nissl stains, toluidine blue-O fluoresces in the red end of the spectrum but such fluorescence quenches upon binding with tissue. Moreover, progressive staining occurs at concentrations low enough to minimise any background fluorescence attributable to non-specific residence of the stain. The bright yellow Di-ASP and vivid green DiO signals are spectrally removed from the red fluorescence of toluidine blue-O. With toluidine blue-O counterstaining, Di-ASP generally offers contrast superior to that with DiI, however, the latter is improved by viewing in a polarised green bright field. Visible Di-ASP emission, although broad, peaks at a more film-sensitive region of the spectrum than that for DiI, thus reducing the photographic exposure required.