Within the hematopoietic compartment, the murine Hlx homeobox gene is expressed in myeloid cells, most prominently in macrophages and granulocytes, and in immature B-lymphoid cells but not in erythroid, mast, or T-lymphoid cells. The level of Hlx mRNA increased with induced differentiation of the promyelocytic lines WEHI-3B and HL-60. To address its biologic action more directly, Hlx expression vectors were introduced into seven mouse hematopoietic cell lines representing several lineages. Although four lines did not tolerate stable Hlx expression, high-level expression was achieved in the early myeloid line FDC-P1 and in the immature T-cell lines Tikaut and WEHI-707. Overexpression of Hlx in FDC-P1 cells downregulated two markers of myeloid immaturity, Thy-1 and CD34, and also promoted changes in cellular and colony morphology, indicative of limited differentiation. Ectopic Hlx expression in the T-cell lines induced changes in cellular and colony morphology and adhesiveness, concomitant with decreased expression of adhesion molecules ICAM-1 (CD54) and Pgp-1 (CD44) and increased expression of heat-stable antigen. These results implicate Hlx in the control of myeloid maturation and the regulation of lymphoid adhesion processes.