The first synthetic estrogen.

of data, statistical analysis, etc. However, we now consider that biological factors, of which we are unaware and for which we have not controlled, have the potential to exert developmental effects on testis weight which are at least as great as the maximum effects that can be induced by the addition of a potent estrogen (DES) to the mother's drinking water during pregnancy and lactation. This conclusion, and our other experiences outlined above, have obvious relevance to the ongoing debate regarding the design and application of in vivo tests for the detection of adverse effects ofhormone disruptors. We consider it our scientific responsibility to bring these matters to the attention of all those involved in this area. Some of the results summarized in the letter above were part of work supported by contract BMH4-CT96-0314 from the European Union.