The mechanism of conjugation of estradiol with either albumin or various thiols in the presence of horseradish peroxidase was studied and these two systems were shown to differ in a number of ways. With albumin, the reaction was slow, required Mn2+, and was inhibited by catalase but not iodoacetamide. In contrast, the reaction with thiols was rapid, did not require Mn2+, and was relatively insensitive to catalase. The yield of conjugates with different thiols and the effect of various inhibitors were also determined. The nature of the water-soluble steroid conjugate was investigated and the product obtained with glutathione shown to differ from that formed by rat liver microsomes. The possible extension of this model system to the situation existing in estrogen-stimulated uteri is discussed.