Flow cytometric analysis of human peripheral blood leukocytes has typically been achieved by staining multiple aliquots of the same sample with fluorescent reagents specific for cell subsets of interest. Spectrally discrete fluorochrome tags have been developed for applications in which identification of multiple subsets (e.g., T and B cells) or of subsets not uniquely identified by a single reagent (e.g., activated T cells) requires use of multiple reagents per aliquot. Extension of this approach to more than two reagents per aliquot has led to multicolor methods requiring dual laser excitation and complex instrumentation. We describe an alternative two-color method using commercially available reagents that allows simultaneous identification of five discrete immune cell subsets using only a single excitation source. The technique uses dilution of commercial fluorochrome-labeled reagents with competing unlabeled reagents to selectively produce discrete fluorescence intensity profiles for cell subsets that would otherwise display overlapping or indistinguishable profiles when stained with reagents bearing the same fluorochrome. For example, the fluorescence intensity of phycoerythrin-labeled helper T (Th) cells can be adjusted to be distinct from that of phycoerythrin-labeled suppressor T (Ts) cells. Extending this technique to two colors, we have used a combination of seven different monoclonal antibodies to simultaneously quantify Th, Ts, B cells, natural killer cells, and monocytes in a single aliquot. An additional advantage of this approach is the ability to more accurately quantify "null" cells. Adjustment of fluorescence intensity profiles of different cell subsets by this method is applicable to flow cytometric analysis of a wide variety of cell types. The technique significantly extends the analytical capacity of flow cytometry without significantly increasing the complexity of the instrumentation required.