We and others have shown previously that retinoic acid (RA) selectively inhibits the growth of estrogen receptor (ER)-positive human breast carcinoma (HBC) cells and ER-negative cells are refractory to RA inhibition of growth. The ER-negative cells inherently express lower levels of RAR alpha and retinoic acid response element (RARE)-mediated RA-induced CAT activity. In this study we report that when ER-negative MDA-MB-231 cells were transfected with the ER gene they not only expressed higher levels of RAR alpha and RARE-mediated RA-induced CAT gene expression, but their growth was not inhibited by RA. Estrogen enhanced RAR alpha gene expression not only in established ER-positive cell lines but also in ER-transfected MDA-MB-231 cells. The estrogen effect appears to be direct and at the gene transcription level since it did not alter the stability of RAR alpha mRNA and cycloheximide failed to block estrogen-mediated enhancement of RAR alpha gene expression. Our data strongly suggest that ER-mediated enhancement of RAR alpha levels plays an important role in RA inhibition of HBC growth. In addition, we also report here that HBC cells appear to express a unique isoform(s) of RAR alpha which was detected only when the full-length RAR alpha cDNA was used as a probe; the RAR alpha 1 and RAR alpha 2 specific probes failed to hybridize with the HBC specific RAR alpha message.