Estrogen receptors of primary breast cancer tumors were determined by sedimentation analysis. Positive tumors from pre- and post-menopausal patients were those that contained estrogen receptor content greater than 250 and greater than 750 fmoles per g wet weight tissue, respectively. A positive correlation of patients' age with increasing incidence of estrogen receptor positive tumors was found; test for linear trend significance z = 5.38 (P less than 0.001). One-sixth of those less than 29

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... s had positive tumors, 27 percent in the age group 40 to 49 years, 46 percent in those 70 to 79 years, and 62 percent in those 80 years or older.