Retinoids mediate their actions via RARs (retinoic acid receptors) and RXRs (retinoid X receptors). Each class of these nuclear retinoid receptors is further subdivided into three species, namely α, β, and γ. Recent studies demonstrate that estrogen receptor (ER)-positive human breast carcinoma (HBC) cell lines and tumor samples exhibit significantly higher levels of RARα than their ER-negative counterparts. ER-positive HBC cell lines are sensitive to, and ER-negative cell lines are resistant to, growth inhibitory effects of retinoic acid (RA). We previously demonstrated that the expression of functional ERs in an established ER-negative cell line resulted in higher levels of RARα and sensitivity to growth inhibition by RA. To further investigate the major role of RARα in retinoid-mediated inhibition of growth, we transfected RARα cDNA in two RA-resistant ER-negative HBC cell lines. Analyses of different clonal populations of RARα transfectants from each cell line revealed growth inhibition by retinoids. Utilizing RAR- and RXR-class selective retinoids, we further demonstrated that only the RARα- selective retinoids mediated the growth inhibition in these cells, while the RXR-selective retinoids were biologically inert. We thus provide evidence that the molecular mechanisms of retinoid inhibition of HBC proliferation predominantly involve RARα.