Notch signaling regulates multiple cell fate decisions by hematopoietic precursors. To address whether different amounts of Notch ligand influence lineage choices, we cultured murine bone marrow lin⁻Sca-1⁺c-kit⁺ cells with increasing densities of immobilized Delta1^ext-IgG consisting of the extracellular domain of Delta1 fused to the Fc domain of human IgG₁. We found that relatively lower densities of Delta1^ext-IgG enhanced the generation of Sca-1⁺c-kit⁺ cells, Thy1⁺CD25⁺ early T cell precursors, and B220⁺CD43^−/lo cells that, when cocultured with OP9 stroma cells, differentiated into CD19⁺ early B cell precursors. Higher densities of Delta1^ext-IgG also enhanced the generation of Sca-1⁺c-kit⁺ precursor cells and promoted the development of Thy1⁺CD25⁺ cells, but inhibited the development of B220⁺CD43^−/lo cells. Analyses of further isolated precursor populations suggested that the enhanced generation of T and B cell precursors resulted from the effects on multipotent rather than lymphoid-committed precursors. The results demonstrate the density-dependent effects of Delta1 on fate decisions of hematopoietic precursors at multiple maturational stages and substantiate the previously unrecognized ability of Delta1 to enhance the development of both early B and T precursor cells.