Whereas the characterization of B lymphoid progenitors has been facilitated by the identification of lineage- nd stage-specific surface markers, the continued identification of differentially expressed proteins increases our capacity to explore normal and malignant B cell development. To identify novel surface markers with stage-specific expression patterns, we explored the reactivity of CD19(+) B cell progenitor cells to Abs targeted to 176 surface proteins. Markers with stage-specific expression were identified using a transgenic reporter gene system subdividing the B cell progenitors into four surface IgM(-) stages. This approach affirmed the utility of known stage-specific markers, as well as identifying additional proteins that selectively marked defined stages of B cell development. Among the stage-specific markers were the cell adhesion proteins CD49E, CD11A, and CD54 that are highly expressed selectively on the most immature progenitors. This work identifies a set of novel stage-specific surface markers that can be used as a complement to the classical staining protocols to explore B lymphocyte development.