Development of a new fixation procedure allowed flow-cytometric analysis of nuclear and other intracellular antigens in acute lymphatic leukemia (ALL). A short fixation of the cells with buffered formaldehyde acetone (BFA) rendered the cell membrane permeable, allowing the monoclonal antibodies (MoAbs) to penetrate the cell. Through this method, a rapid analysis of intracellular antigens, specific for acute lymphatic leukemia [such as terminal deoxynucleotidyl transferase (TdT), immunoglobulin M (IgM) heavy chain, and antigens recognized by the CD22 or CD3 MoAbs) was performed by flow cytometry. The surface antigens remained intact after this fixation procedure, enabling simultaneous detection of membrane and intracellular antigens. The binding of biotinylated antibodies against several B- and T-lymphoid membrane antigens was detected with streptavidin-phycoerythrin (red fluorescence), whereas the intracellular antigens were stained with FITC-labeled polyclonal antibodies, or indirectly with FITC-labeled goat anti-mouse IgG (green fluorescence). Through this combination of markers, minor cell populations can be detected and a rapid and quantitative immunodiagnosis can be performed.