The active vitamin D3 metabolite 1,25-dihydroxycholecalciferol (1,25(OH)2D3) acts as an antiproliferative and differentiating agent for the monoblastoid cell line U937 and as an important immunologic mediator implicated particularly in the function of cells belonging to the monocyte/macrophage lineage. These effects are controlled by the vitamin D receptor (VDR), a member of the steroid hormone receptor family. The objective of this study was to develop U937 transfectants expressing antisense VDR mRNA, and to use these to examine the role of 1,25(OH)2D3-VDR interaction in this lineage. A 2-kb VDR cDNA insert (including the complete VDR coding region) was cloned in an antisense orientation into the EBV episomal vector pMEP4 under the control of an inducible promoter and transfected into U937. The resultant cell line, DH42, was hygromycin resistant, contained VDR cDNA, expressed fewer VDRs than controls, and showed a substantial decrease in antiproliferative response to 1,25(OH)2D3. However, 1,25(OH)2D3 increased the number of cells expressing macrophage cell surface Ags, including CD14 and CD11b. A subpopulation of smaller cells did not express the differentiation markers after cadmium stimulation. Cell cycle analysis showed shifts in the distribution of cells from G1 to S phase, which were more pronounced after cadmium treatment. A considerable proportion of cells were outside the cycle and DNA fragmentation confirmed apoptosis. Thus, the functional outcome of the VDR antisense transfection suggests that in the myelomonocytic lineage, VDR expression may act as a protective mechanism against programmed cell death.