HLA-DR is one of the markers associated with hematopoietic cell differentiation, since expression of this molecule is modulated throughout hematopoiesis. We have previously described and cloned the gene encoding factor IK, which inhibits both interferon gamma (IFN-gamma)-induced and constitutive HLA-DR expression. The current study demonstrates that IK gene transcripts are present in CD34+ cells purified from human umbilical cord blood. IK expression increased and was therefore inversely correlated with the gradual loss of HLA-DR during growth factor-induced CD34+ cell proliferation and differentiation. To study the possible role of IK in hematopoiesis, antisense probes were used. IK expression was specifically inhibited by an antisense oligodeoxynucleotide containing two phosphorothioate internucleotide linkages at each of the 3' and 5' ends and corresponding to the initiation site of IK mRNA. A control oligonucleotide was also tested in parallel. A specific decrease of IK transcripts was correlated with an increase of HLA-DR antigen expression level. In colony-forming assays, IK antisense oligonucleotide inhibited colony formation by multilineage early erythroid and granulomonocytic CD34+ progenitors. The mean colony size was decreased 70% by IK antisense oligonucleotide in comparison to controls. These results provide evidence that the IK molecule participates in the regulation of HLA-DR expression on hematopoietic cells and plays a role in growth factor-dependent CD34+ cell proliferation and differentiation by modulating HLA-DR expression.