Solid-phase-based immunoassays have traditionally been plagued by nonspecific binding to the solid phase and by slow reaction kinetics relative to reactants that are free to diffuse in solution. We have developed two novel immunoassays in which the solid phase is generated in situ after the specific binding reaction has occurred, thereby enhancing reaction kinetics and minimizing the opportunities for non-specific binding. In the first system, the capture antibody is conjugated to an organic monomer, polymerization of which to form insoluble polymer particles is initiated by a reaction involving free radicals. The amount of signal-labeled antibody incorporated into the resulting particles is directly proportional to the concentration of antigen. The principle is illustrated for the simultaneous assay of IgG and IgM in a single sample. In the second system, capture antibody is conjugated to a polymer, the solubility of which is a function of temperature. Specific binding is conducted below the critical solution temperature of the polymer, which is then separated from solution by increasing the temperature above the critical temperature. The incorporation of signal-labeled antibody into the precipitated polymer is directly proportional to the concentration of antigen. This principle is illustrated for the assay of hepatitis B surface antigen and Chlamydia trachomatis.