The dissociation constant for the complex between cyclic nucleotide phosphodiesterase and fully-liganded calmodulin at pH 7.0, 25 C, has been redetermined from six sets of data to be 10 to the minus eleven M. (2) The mechanism of activation of proteinase B in yeast at pH 5 involves the loss of in hibitory effect of proteinase B inhibitor, I(B), due to a slow irreversible conformational rearangement. Isoelectrofocusing experiments revealed that I(B) can exist in four forms which differ in susceptibility to loss of inhibitory effect by pH 5 treatment. (3) The theory for the continuous variation method for the determination of binding stoichiometry has been extended to proteins exhibiting cooperative binding effects. The theory is applicable to both the sequential model of Adair and the concerted model of Monod.