PharmingenStain Buffer (BSA) can be used for the immunofluorescent staining of single-cell suspensions prepared from either lymphoid tissues, bone marrow, peripheral blood, or cultured cells. PharmingenStain Buffer (BSA) is useful for the dilution and application of fluorescent reagents as well as for the suspension, washing, and storage of cells destined for flow cytometric analysis (or fluorescence microscopy). Based on previous descriptions of staining media, PharmingenStain Buffer (BSA) was formulated as a neutral pH (pH 7.4)-buffered salt solution (i.e., DPBS) that is supplemented with 0.2% (w/v) bovine serum albumin (BSA) proteins. As such, PharmingenStain Buffer (BSA) is designed to maintain cell viability and maximize fluorescence signal intensities generated by pH-sensitive fluorochromes, e.g., fluoresceine isothiocyanate (FITC) . PharmingenStain Buffer (BSA) is useful for staining cells with biotinylated antibodies and fluorochrome-conjugated avidins because this staining medium contains no biotin . When present in staining media, free biotin can interfere with the binding of fluorescent-avidins to biotinylated antibodies that have complexed with their cognate, cell-associated antigens. Moreover, free biotin can bind to cells and contribute to an increase in the non-specific, background staining of cells that are exposed to fluorochrome-conjugated avidins. PharmingenStain Buffer (BSA) contains the metabolic inhibitor, sodium azide (NaN3). NaN3 inhibits the potential redistribution of cell surface antigens (e.g., due to shedding or internalization) caused by antibody crosslinking. NaN3 (in combination with maintenance of cold ambient temperatures) thereby prevents the potential loss of fluorescent signal intensities generated by immunofluorescently-stained cells during subsequent flow cytometric analysis(or fluorescence microscopy).