Estrogens are believed to play a crucial role in growth regulation and differentiation of the normal endometrial tissue as well as in the carcinogenesis of the endometrium. Therefore, the influence of estrogens and antiestrogens on gene expression in the estrogen receptor-positive rat endometrial adenocarcinoma cell line RUCA-I was investigated. Differentially expressed genes were detected by differential display PCR of RNA of untreated, estradiol-treated and antiestrogen-treated RUCA-I cells. By means of the PCR technique, 14 differentially expressed fragments could be detected. Three of these 14 differentially expressed fragments were confirmed by Northern blotting. The steady state mRNA levels of the three gene fragments named AH41, AH42 and AH44 were downregulated by the antiestrogen ICI 164384. Further characterization revealed that the fragment AH41 is not expressed in stromal cells but in the human and rodent epithelial cell lines, BG-1 and RUCA-II. A comparison of the cDNA sequence of fragment AH41 with the EMBL database showed no high homology to known genes. Therefore, fragment AH41 has to be regarded as a fragment of a novel, estradiol-sensitive gene.