B cell Ag receptor (BCR) signaling changes dramatically during B cell development, resulting in activation in mature B cells and apoptosis, receptor editing, or anergy in immature B cells. BCR signaling in mature B cells was shown to be initiated by the translocation of the BCR into cholesterol- and sphingolipid-enriched membrane microdomains that include the Src family kinase Lyn and exclude the phosphatase CD45. Subsequently the BCR is rapidly internalized into the cell. Here we show that the BCR in the immature B cell line, WEHI-231, does not translocate into lipid rafts following cross-linking nor is the BCR rapidly internalized. The immature BCR initiates signaling from outside lipid rafts as evidenced by the immediate induction of an array of phosphoproteins and subsequent apoptosis. The failure of the BCR in immature B cells to enter lipid rafts may contribute to the dramatic difference in the outcome of signaling in mature and immature B cells.