In this report we investigate the role of homeobox genes of the Antennapedia class in adult erythropoiesis, the process by which erythrocytes are formed in the bone marrow. We initially identified Hox genes expressed in mouse erythroleukemia cells by a PCR technique using degenerate primers capable of detecting most of the known genes in Hox clusters 1-4. Four different transcripts, Hox-3.3, -3.5, -3.6, and -4.3 were identified. An antisense oligonucleotide directed against HOX3C (human homologue of Hox-3.3) was used to study the effect of the colony formation by human erythroid progenitor cells. The oligonucleotide inhibited the formation of colony-forming unit erythroid-derived colonies but did not affect the size or degree of hemoglobinization. The more primitive erythroid burst-forming unit colonies or myeloid colonies were not affected. These results show that Hox-3.3 is involved in an early step in the proliferation of the erythroid colony-forming unit subset of progenitor cells.