This Test Guideline describes an in vitro assay, which provides concentration-response data for substances with in vitro ER agonist and antagonist activity. The test system utilises the BG1Luc4E2 cell line derived from a human ovarian adenocarcinoma and stably transfected with a ER responsive luciferase reporter gene. This cell line can evaluate TA mediated by ER alpha and ER beta. The cells are plated into 96-well plate and exposed to 7 (range finder tests) and 11 (comprehensive test) non-cytotoxic concentrations of the test chemical for 19-24 hours to induce the reporter gene product (luciferase). Its activity is measured in a luminometer. Acceptance or rejection of a test is based on the evaluation of reference standard and control results from each experiment conducted on a 96-well plate. A positive response is identified by a concentration–response curve containing at least three points with non-overlapping error bars, as well as a change in amplitude (normalized relative light unit) of at least 20 % of the maximal value for the reference substance (17beta-estradiol for the agonist assay, raloxifene HCL/ 17beta-estradiol for the antagonist assay).