Homeobox proteins are sequence-specific DNA-binding proteins initially implicated in the control of gene expression in developing tissues; however, there is increasing evidence that these proteins are important in gene regulation in adult tissues. A cDNA for the homeobox gene HOX-2.3 was isolated from an adult human B-lymphocyte cDNA library. Northern blot analysis showed expression of a 1.1 and a 1.6 kb messenger RNA (mRNA) in a human B-cell line. RNase protection assays demonstrated variable expression in both human B- and T-cell lines. Virally transformed and nontransformed lymphocyte cell lines expressed HOX-2.3 transcripts. Essentially no transcripts were found in unactivated normal B and T lymphocytes; however, B-cell activation with Staphylococcus aureus Cowan strain I and phorbol myristate acetate (PMA) or T-cell activation with phytohemagglutinin and PMA were accompanied by a rapid induction of HOX-2.3 expression even in the presence of the protein synthesis inhibitor, cycloheximide. In situ hybridization was performed to examine HOX-2.3 expression in lymphoid tissues. HOX-2.3 mRNA was detected in the thymic cortex from an 8-year-old child, in the germinal centers in adult tonsil, and in a limited number of hematopoietic cells from the bone marrow. These findings suggest the involvement of HOX-2.3 in regulating gene transcription not only in developing tissues but in hematopoietic and lymphoid tissues as well.