The generation of lymphoid cells in mice depends on the function of the Ikaros protein. Ikaros has been characterized as a lymphoid-restricted, zinc- finger transcription factor that is derived from an alternatively spliced message. Ikaros knockout mice have defects in multiple cell lineages, raising the question of whether the protein regulates multiple committed progenitors and/or multipotent stern cells. To address this issue, we examined Ikaros expression in purified populations of multipotent cells and more committed progenitors. We found that the DNA-binding isoforms of Ikaros were localized in the nucleus of the most primitive hematopoietic stem cell subset. Changes in the RNA splicing pattern of Ikaros occurred at two stages: (i) as long- term self-renewing stem cells differentiated into short-term self-renewing stem cells and (ii) as non-self-renewing multipotent progenitors differentiated into lymphoid-committed progenitors. Unexpectedly, we found Ikaros localized to heterochromatin in Abelson-transformed pre-B lymphocytes by using immunogold electron microscopy. These observations suggest a complex role for Ikaros in lymphoid development.