Members of the large Eph family of receptor tyrosine kinases (RTKs) display temporally and spatially restricted expression patterns during embryogenesis, suggesting a role in various developmental processes. We have begun to investigate the expression of members of this receptor family during human hematopoiesis, in particular B lymphopoiesis. Expression of Eph RTKs in cells of the B-lymphoid lineage was assessed by using degenerate oligonucleotide primers based on stretches of conserved nucleic acid sequences in members of the Eph family. First, the content of Eph-family RTKs was assessed in freshly sorted fetal bone marrow pro–B cells. This population was found to harbor transcripts of the Hek8 and Hek11 members of this gene family. Subsequent analysis of expression of these genes in B cells representing various differentiation and ontogenic stages showed that the Hek8 transcript is constitutively present in all fetal and adult B-lineage cells, with high levels of expression in peripheral blood B cells. In contrast, the Hek11 transcript was exclusively found in fetal bone marrow pro–B cells and pre–B cells, but not in more mature fetal B-lineage cells. All adult B-lineage cells, from early pro–B cells to end-stage plasma cells, lacked Hek11 transcripts. The developmentally regulated expression of Hek11 during fetal B lymphopoiesis suggests a role for this gene in pre/pro–B cell expansion and/or differentiation and defines a difference in progenitor B cell populations isolated from fetal versus adult human bone marrow.