Human chorionic gonadotropin (hCG) inhibits the growth of human breast epithelial cells MCF-7. Differential display was utilized for analyzing RNA of hCG-treated MCF-7 cells in order to determine whether this effect was mediated by specific gene activation or repression. Three cDNA clones, 19, 29 and 44, were differentially expressed in treated cells. Clone 44, whose sequence matched a gene expressed in dexamethasone-treated T-cell hybridoma, was increased by approximately 4-fold, while clones 19 and 29 were markedly reduced by hCG treatment. These latter ones were considered to be novel genes, since no homology was found in the gene-bank. Our results indicate that hCG influences gene expression in mammary epithelial cells, supporting its potential role in breast cancer prevention and therapy.