Levels of insulin, estrone, estradiol, estriol, progesterone, and testosterone were determined in commercial fetal calf, newborn, calf, and bovine sera. Hormone levels in the sera varied significantly, with a range from 1–23 times. Marked difference in the ratios of the hormone concentrations was evident. With the increasing interest in organ culture, especially of endocrine-responsive organs, and since tissue response is the result of a net effect of hormonal ratios as well as concentrations, the results suggest that hormonal profiling and standardization of hormone concentrations be included as part of the quality control testing of sera used in tissue cultures.