A cDNA library has been constructed from the poly(A)+ mRNA of oestrogen-stimulated ZR-75-1 human breast cancer cells. Screening by differential hybridization has identified eight clones which are stimulated between 4- and 16-fold by oestrogen. Two clones (pLIV-1) that are stimulated 4-fold, hybridize to three different mRNA species. A further five recombinants encode for a mRNA 600 bp long which is induced greater than 16-fold and have been shown to cross-hybridize to the oestrogen-responsive clone, pS2, isolated from the MCF-7 breast cancer cell line. Oestradiol was shown to be without detectable effect upon the expression of mRNA for dihydrofolate reductase, which is reported to be oestrogen regulated in MCF-7 cells. Actin gene expression is also unresponsive to oestradiol in ZR-75-1 cells. These results suggest that pLIV-1 represents a previously unidentified mRNA that may be involved in the oestrogen-regulated growth of ZR-75-1 human breast cancer cells.