at high concentrations at which test com? pounds were precipitated, cells indicated an abnormal response in the luciferase activity of control plasmids and in morphology (data not shown). To construct a stable assay sys? tem, we used HeLa cells transfected with an hERa expression plasmid derived from nor? mal human liver ERa. In this assay system, styrene dimers and trimers did not show any increase in E2-dependent luciferase tran? scription activity. These results agreed with the result of the ER binding assay. We presume that styrene dimers and trimers had no binding affinity to ER and they did not affect E2-dependent transcription. As a result, in our comparison of three ER binding assays using estrogenic and nonestrogenic compounds, it appeared that Method RI and Method A were useful for evaluating binding affinity for the ER, but Method B, similar to the method of Ohyama et al. (10), tended to indicate false-positives in high concentrations in which test chemicals were insoluble; this reduced the specificity of ER to E2. Based on our present results and previous reports (2-7), we found no endocrine-disrupting activities in styrene dimers and trimers eluted from polystyrene-containing instant noodle containers.