The aggregation equilibria of Escherichia coli RNA polymerase core and holoenzyme have been studied by velocity sedimentation as a function of [NaCl] both in the presence and in the absence of MgCl2. Effects of other anions (F- and I-), pH, and temperature have also been examined. Diffusion coefficients obtained by quasi-elastic light scattering (QLS) at high and low salt concentrations were used in conjunction with sedimentation coefficients under these conditions to obtain molecular weights of the protomer and aggregates of the core enzyme. At low salt concentration, core aggregates to a tetramer in the absence of MgCl2 and to an octamer in the presence of MgCl2. Some ambiguity exists in the interpretation of the sedimentation and QLS data for holoenzyme. The sedimentation results are consistent with the formation of dimers at low salt, both in the presence and in the absence of MgCl2. In all cases, equilibrium constants were calculated assuming a simple monomer--j-mer stoichiometry. These equilibrium constants are extremely sensitive functions of the concentration and type of monovalent anion. In Cl-, aggregation of both core and holoenzyme begins abruptly when the salt concentration is reduced below approximately 0.2 M (at a protein concentration of approximately 0.30 mg/mL); for core, substitution of I- for Cl- suppresses aggregation while F- enhances aggregation at a fixed anion concentration. No specific effect of monovalent cations (Na+, NH4+) is observed; Mg2+ has no effect on holoenzyme dimerization and has little effect on the salt range of core aggregation, though the stoichiometries of the core aggregates in the presence and absence of Mg2+ differ. Anion effects on these equilibria were modeled by assuming that a class of anion-binding sites on the protomer is not present in the aggregate, so that anion release accompanies aggregation. Analytical expressions for several models of the effect of anions on the aggregation equilibria were derived by using the method of binding polynomials. The salt dependence of the aggregation equilibria in the absence of Mg2+ appears inconsistent with a model in which the anion-binding sites on the protomer are independent (noncooperative), but it is well described by a model in which anion binding to the protomers occurs in a completely cooperative manner. The molecular basis of this apparent cooperative effect of anions on the aggregation equilibria is proposed to be an allosteric effect of anions on conformational equilibria of the protomers of core polymerase and the holoenzyme. Implications of such a salt-dependent conformational transition for the DNA-binding interactions of the enzyme are considered.