Selective reduction of native disulfide bonds in immunoglobulins is one of the best methods for introducing reactive groups on to the protein surface. Additionally, the thiol groups so generated may allow oriented conjugation at a specific site of the immunoglobulin. Solid-phase reducing agents have many advantages over soluble ones (including ease of separation of excess reagent from reduced protein by filtration, and the potential for regeneration and multiple reuse). In this work we report a comparative study of the reduction of rabbit IgG and its F(ab')(2) fragments, with mercaptohydroxypropylether-agarose (thiopropyl-agarose), a solid phase reducing agent, and dithiothreitol. The effect of different parameters on the process, such as the amount of reducing agent, incubation period, and temperature, was assessed by titration of thiol groups and SDS-PAGE analysis. Optimized reduction with thio-propyl-agarose introduced six thiol groups in the F(ab')(2) fragment (mollmol). Native IgG was less reactive, probably due to steric effects, as only an average of three thiol groups were introduced. However, by increasing reaction temperature from 22 to 37 degrees C, six thiol groups could be introduced in native IgG (mollmol). Reduction with dithiothreitol also introduced six thiol groups in F(ah')2 fragments (mollmol) but led to higher thiol content for the whole IgG. These results demonstrated that thiopropyl-agarose can be a very useful tool for exercising more precise control over the reduction treatment, and for selecting which disulfide bridges are to be broken. After 6 h incubation with reducing agent containing 8 and 16 mu moles SH per mg of protein, the resulting reduced IgG retained the same biological activity as the native immunoglobulin. The controlled modification of native disulfides achieved with thiopropyl-agarose will be useful for the development of soluble and insoluble immunoglobulin conjugates.