Proliferating cell nuclear Ag (PCNA) is an intranuclear protein involved in DNA replication directed by DNA polymerase delta and is the target Ag of autoantibodies in some patients with SLE. There is evidence that the epitope on PCNA recognized by human autoantibodies is conformation-dependent and is not a continuous peptide sequence. Thimerosal, a mercury-containing sulfhydryl blocking compound, markedly reduced or abolished the reactivity of this autoantibody-defined PCNA epitope. The thimerosal effect was observed in various Ag-detecting systems including indirect immunofluorescence, immunodiffusion, immunoprecipitation, and flow cytometry. The mechanism of the thimerosal effect appeared to be mediated through free but not readily accessible sulfhydryl group or groups. The sulfhydryl-modification by thimerosal could be reversed by competition with thiol-containing compounds. Experimentally induced mAb to PCNA generated by immunization with purified PCNA have been shown to recognize epitopes that are continuous peptide sequences and these epitopes were not affected by thimerosal. It has been shown that the human autoantibody-defined epitope is related to the function of PCNA because autoantibodies are able to inhibit DNA polymerase delta directed DNA replication, whereas experimentally induced antibodies are not. These studies show that certain sulfhydryl groups in PCNA have a role in determining the antigenicity of the epitope recognized by autoantibody and raise the possibility that certain sulfhydryl groups might also be associated with its function.