The equilibrium [3H]estradiol binding by the partially purified estrogen receptor from calf uteri was measured at 25 degrees C. The Scatchard plot of the binding data showed a convex curve characteristic of positive cooperativity and a Hill coefficient of 1.58 +/- 0.21, at receptor concentrations of 1 to 10 nM. Below a receptor concentration of approximately 0.3 nM the Scatchard plot approached linearity, suggesting that the cooperative interactions are dependent upon a monomer--dimer equilibrium. Trypsin pretreatment of the receptor resulted in a loss of dimer formation and of the cooperative interactions. The positive cooperative characteristics of the estrogen receptor were shown not to be produced by receptor inactivation, failure to complete the [3H]estradiol--receptor equilibrium reaction, or radioimpurity of the [3H]estradiol. These findings indicate that the activated 5S estrogen receptor is a homodimer and that its formation is associated with a positive cooperative estradiol-binding reaction.