Pararosaniline-Feulgen staining of cells in suspension produces nucleus- and chromatin-specific fluorescence as well as color. Experiments were designed to test postulated reaction mechanisms responsible for the fluorescent staining with the nonfluorescent pararosaniline. The reduction in fluorescent-staining intensity by pretreatment of cells with 2.2 x 10-2M K2S2O5 tends to rule out the alkysulfonic acid pathway; conditions favoring the formation of this intermediate reduce staining intensity. The fluorescence enhancement, observed when cells stained in pararosaniline without K2S2O5 are post-treated with K2S2O5, suggests that there is an initial Schiff-base linkage between pararosaniline and an aldehyde of hydrolyzed DNA, and that this linkage is stabilized in the presence of K2S2O5. Microspectrofluorometer measurements of cells stained at various pararosaniline concentrations in 2.2x10-2M K2S2O5, show that the fluorescence emission maximum ranges from about 627 nm at 3.1x10-3 M pararosaniline to about 604 nm at 3.1x10-5M. All of the employed staining protocols appear to produce the same fluorescent product, perhaps a heterocyclic pyronin analog formed from pararosaniline. Flow microfluorometric analysis of cells stained in suspension verified that the relative fluorescence intensity represents relative DNA content. Staining at reduced pararosaniline concentration (3.1x10-4M) reduces the coefficient of variation of the flow microfluorometric histograms, showing that maximum quantitation does not necessarily correlate with maximum staining intensity.