The previous monoclonal antibody labeling method for bone marrow immunoscintigrapy was complicated and laborious for clinical application. Also it showed a relatively low labeling efficiency. To improve this procedure, we compared several direct labeling methods of Tc-99m. 1) The labeling efficiency in the method using gluconate as a transchelator was low (40-70%), but immunoscintigraphy using this radiotracer produced a clear image. 2) To improve labeling efficiency, β- mercaptoethanol was removed after reduction. The labeling efficiency was improved up to 70-80%, but the radioactivity of the blood pool was high. 3) The higest labeling efficiency (〉90%) and best quality images could be obtained by using MDP as a transchelating agent. It did not require additional procedures for separation of labeled antibodies. The immunoreactivity of this antibody was 60%. Residual MDP which can be taken up by the bone could be removed by PD-10 column. The reduced antibodies were stable with a high labeling efficiency (〉90%) for up to 47 days by deep freezing. We concluded that the improved procedure for Tc-99m labeling of anti-NCA-95 monoclonal antibody using MDP as a transchelating agent will be a simple and useful method for clinical application.