Monoclonal proteins directed against nuclear antigens are extremely rare. We describe the use of a double antibody immunofluorescence technique to document the existence of a monoclonal antibody with nuclear antigen activity. The patient was an elderly woman with a small monoclonal protein, but no evidence of a lymphoproliferative or autoimmune disorder. A homogeneous antinuclear antibody (ANA) prompted further studies to determine whether the monoclonal protein was related to the ANA activity. A double-antibody immunofluorescence procedure is described by which the ANA positivity was shown to be entirely due to a monoclonal IgMK protein. It consists of using fluorescein anti-kappa and rhodamine anti-lambda applied to a section of mouse kidney treated with the patient's serum. Documentation of antigenic specificity of monoclonal antibodies by double antibody fluorescence provides a straightforward method that can be utilized by routine clinical laboratories.