The weight-average molecular weight of Clr, an activated serine protease subcomponent of complement Cl, was measured in the presence of widely varying concentrations of Ca2+ and the other serine protease subcomponent, Cls, by utilizing the technique of tracer sedimentation equilibrium. A quantitative model for heteroassociation between the two subcomponents, which takes into account the previously observed Ca(2+)-dependent self-association of Cls, was fit to the combined data at each Ca2+ concentration. The results indicate that Clr, which exists as a dimer under all of the conditions explored in this work, can bind up to two molecules of Cls at both low and high Ca2+ concentrations, but the association constant for binding a single molecule of Cls to dimeric Clr is estimated to increase on the order of a 1000-fold as [Ca2+] increases from 1 nM to 1.0 mM. Heteroassociation of Clr and Cls is favored over self-association of Cls at all conditions. The results clearly indicate the necessity of taking into account a multiplicity of states of association when attempting to understand the equilibrium average properties of a mixture of the two subcomponents and their binding to Clq in solution.