The TEF approach for hexachlorobenzene.

same variety of soybeans grown in different locations (25,26). Also, in closed formula diets containing soybeans, the concentration of soybeans in the diet is not reported and may vary from batch to batch depending upon the supply and demand of dietary ingredients. This could explain why the phytoestrogen content of diets may vary greatly from batch to batch or from the same diet processed by different rodent diet vendors. These findings support the need to use open formula diets prepared from controlled ingredients and the need to monitor these diets for estrogenic substances that affect biological end points. We also agree with Boettger-Tong et al. (1) that rodent diet vendors do not routinely monitor all diets for estrogenic substances including phytoestrogens. Rodent diet vendors producing certified diets for use in comparative estrogenicity and carcinogenic-ity studies should provide the user with a list of substances assayed, including phytoe-strogens, and the results. In our opinion, diets used in comparative estrogenicity or carcinogenicity studies should contain non-detectable levels of estrogenic substances that may alter research results. These diets should be monitored for estrogenic substances and their concentrations reported. The commentary by Boettger-Tong et al. (1) is important because the authors emphasize the important role of the ani-mal's diet, especially phytoestrogens, when conducting animal bioassays for estrogenici-ty or studies that are influenced by in vivo end points of hormone action. We previously reported that rodent diets significantly differ in estrogenic activity and that a standardized diet with minimal estrogenic activity would be desirable for comparative bioassays for estrogenic substances (5). Results from our second study (20) confirm that a standardized open formula diet should be used for studies that are influenced by exogenous estrogens (4,5). Phytoestrogens were not detected in the AIN-76A and the AIN-93M purified casein diets. Therefore, careful consideration should be given to the use of diets such as these when conducting studies that are influenced by exogenous estrogens. We have also shown that a natural ingredient diet can be formulated to contain less than detectable levels of the phytoestro-gens (daidzein and genistein) by omitting soybean and alfalfa meals. The soybean and alfalfa meals were omitted because they may be a source of multiple, yet unidentified, phytoestrogens that may further complicate the interpretation of results from studies which are influenced by exogenous estro-gens. We recommend that studies to determine the effects of dietary phytoestrogens on results of toxicloOgiC investigations are important and …