Nine monoclonal antibodies against null cell ALL cells were obtained by mouse hybridoma technique. Using two antibodies, namely B-1 and D-22, out of these, cell surface antigens on various leukemia cells, normal bone marrow cells, spleen cells and cultured cells were analysed by means of flow cytometry. B-1 antibody was found to have reactivities against null cell ALL cells as well as some CML-blastic crisis cells. The antibody reacted against some cultured lymphoid cell lines and also several percent of bone marrow cells. The reactivities of B-1 antibody were quite similar to those of c-ALL by Greaves et al and J-5 by Ritz et al. Radioimmunoprecipitation experiment suggested that B-1 antibody define the same antigen as the other two do. Meanwhile, D-22 antibody reacted against null cell ALL cells, but did not react against any other types of leukemia cells or any other cultured cell lines, or bone marrow cells. Thus D-22 antibody was considered to have higher specificity for null cell ALL cells than any other antibodies reported. On the other hand, the analyses by flow cytometry were well correlated with the analysis by immune adherence assay and by immunofluorescent microscopy. Besides, the analysis by flow cytometry provided us precise quantitative data and histogram pattern of positive cells. Furthermore, detailed examination of subpopulation could be done using cell sorter or other biological features could be analysed simultaneously using another fluorescent dye. Although, it has a few limitations, flow cytometry could take over other methods which are currently used in this field. Finally, the possibility of this method for the clinical diagnosis of leukemia was discussed.