AbstractThe identification and quantitation of intracytoplasmic immunoglobulins has become a useful tool in the clinical laboratory evaluation of neoplastic cells from patients with lymphocytic leukemia. Currently, the vast majority of these studies are made by fluorescent microscopy using cytocentrifuge preparations. In the present communication we report a new procedure for rapid flow cytofluorometric analysis of intra‐cytoplasmic immunoglobulin. This technique, utilizing a primary fixation step, permits maximal penetration of labeled antibody while preserving plasma membrane integrity and high retention of intracellular target proteins. These attributes contribute to a rapid yet accurate procedure for quantitating intracellular immunoglobulins which is potentially superior to any method currently utilized. Although the emphasis of this investigation has been on the labeling of intra‐cytoplasmic μ chain, continuing studies in our laboratory indicate that the technique provides a sensitive method for detecting a variety of intracellular antigens.