Soluble oligovalent antigen--antibody complexes were prepared and analysed by ultracentrifugation in order to study the effect of the combining ratio, antigen valence and concentration upon the size and molecular composition of the composition of the complexes. Fluorescein (F) conjugates of rabbit serum albumin (RSA) and thyroglobulin (RTg) were combined with high affinity rabbit anti-F antibodies to form soluble complexes. The effect of the combining ratio paralleled findings in precipitating systems in that the largest soluble complexes were found at equimolarity and mild molar antibody excess. Tetravalent antigen formed precipitates at combining ratios near equimolarity, whereas trivalent antigens failed to precipitate at similar concentrations. Complexes prepared near equimolarity were most sensitive to changes in concentration, higher concentrations leading to larger complexes. The Ab/Ag ratios of different-size complexes in the same preparation were remarkably similar. This ratio was dependent on the antibody--antigen combining ratio, was limited by antigen valence and was not affected by concentration differences. The data support the hypothesis that soluble complexes are formed in two steps. First, antigen and antibody combine to form subunits whose Ab/Ag ratio is determined by the combining ratio and antigen valence. These subunits then combine to form larger complexes in a manner analogous to polymerization.