Optical brighteners of the diaminostilbene type are fluorescent dyes which are popular diagnostic tools in the mycology laboratory. While these dyes are conventionally used for the in vitro diagnosis of mycoses, their low toxicity and chemical reactivity have led us to investigate their potential use for in vivo staining of fungal elements in mycotic tissue. In mice we have established deep-seated candidiasis, cryptococcosis, aspergillosis and zygomycosis, as well as coccidioidomycosis, histoplasmosis and blastomycosis. After establishment of infection, which mostly required immunosuppression, a single dose of 100 microl of an aqueous solution (2.2 x 10(-4) M) of the optical brightener Blankophor P fluessig (4,4'-Bis [(4-anilino-6-substituted-1,3,5-triazine-2-yl) amino] stilbene-2,2'-disulfonic acid) was injected by the tail vein and the animals were sacrificed 1 h later. Sections of freshly prepared target organs were directly subjected to epifluorescence microscopy using an appropriate filter kit. In most cases, fluorescent fungal elements could be detected in the murine tissue. There was little evidence for uptake of the dye by non-infected tissues. It is suggested that radioactive labeling may render parenteral Blankophor suitable for radiographic localization of deep-seated mycotic foci in the host.