It has been proposed that the intestinal microflora plays an active role in the biosynthesis and metabolism of estrogens. The intestinal microflora could form these estrogens through metabolism of estrogen conjugates secreted into the small intestine via the bile or by biosynthesis from other steroids, sterols or bile acids present in the intestinal lumen. It has been proposed that alterations in diet lead to significant alterations in the intestinal microflora, leading to altered biosynthesis and metabolism of estrogens in the gut. This difference is reflected by a different pattern of estrogens in the blood and urine. Two possible mechanisms for the difference in the formation of estrogens in the different groups are: (a) A difference in the intestinal microflora capable of biosynthesis and/or metabolism of estrogens; (b) increased amounts of sterols, neutral steroids or bile acids in the intestinal lumen for conversion to estrogens by the intestinal microflora. Either of these mechanisms would lead to an alteration in the intraluminal pattern of estrogens available for reabsorption. We propose to use standard in vitro and in vivo techniques to demonstrate that there is a difference in the intraluminal pattern of estrogens after significant alteration of the diet and that this difference is reflected in vivo by altered blood and urine levels. If these relationships can be shown to exist, they may explain why the pattern of estrogens is different in individuals at high risk of breast cancer. Further, an important environmental factor (diet) in the induction of breast carcinoma may be identified.