During early B lymphopoiesis, developing B cells maintain lineage commitment despite the local presence of myeloid lineage-promoting cytokines such as GM-CSF and IL-3. Previous observations suggest that the B cell-specific transcription factor Pax5A (paired box 5A transcription factor) plays a role in maintaining B cell lineage commitment by limiting expansion and survival of early IL-3/GM-CSF-dependent myeloid lineage cells. To define a mechanism by which Pax5A can exert these inhibitory effects on myeloid lineage differentiation, an inducible form of the Pax5A protein was expressed in the myeloid cell line FDC-P1. This cell line models myeloid progenitors in that it responds to the survival and growth-potentiating effects of IL-3 and GM-CSF. We observed that enforced expression of Pax5A selectively suppressed proliferation in response to GM-CSF, but not IL-3. This effect was associated with specific down-regulation of GM-CSFR α-chain, but not β-chain expression. These data provide a molecular mechanism to enforce commitment to the B cell lineage despite the presence of GM-CSF, a factor that has been shown to convert early developing B cells to the myeloid lineage. Furthermore, they indicate a role for B cell Pax5A expression in maintaining rather than directing commitment to the B cell lineage.