The objectives of this project were to evaluate and make recommendations for improving the mouse bioassay (NIH, Std. No. 2C, 12-8-72) for detecting estrogenic activity in animal diets. The test diets from 9 of 9 bioassays were reported to be non-estrogenic by a contract testing laboratory. When results were analyzed, 3 of 9 bioassays were determined to be invalid. The mean uterine weight to the terminal body weight ratio for the positive control mice was not 1.5 times as great as that of the negative control mice, as required for a valid bioassay. Uterine weights for the negative control mice of the 3 invalid bioassays were significantly heavier (P less than .01) than the uterine weights for the negative control mice of the 6 valid bioassays. These higher uterine weights in some of the negative control mice may have been due to endogenous hormone activity, probably as a result of failure to control test animal age within the recommended range (14-17 days). Uterine weight increases caused by hormonal activity may occur as early as 22 days of age in some female mice; hence, bioassay studies should be concluded before uterine weight increases due to endogenous hormone begin. To establish the lower limit of useable age, female mice were weaned at 14, 16, 18, or 20 days of age and body growth curves determined. Mice weaned at 14 days and weighed 2 days later lost weight; however, by 24 days of age, they were indistinguishable from mice weaned at 16, 18, or 20 days. Female mice weaned at 16 days of age were chosen to determine the body : uterine weight ratio growth curves. These females showed increases with minor variations in weights from 16-22 days of age. After 22 days of age rapid uterine growth occurred in some mice resulting in large variations in uterine weights from 22-32 days. These results suggest that data collected beyond the 22 day age are misleading in terms of the bioassay, and that only carefully controlled age ranges of weanling mice are suitable for the bioassay.