Although flow cytometry is not yet widely used for diagnostic purposes, it provides a powerful tool for investigating structural and functional properties of immune-responsive cells. By far, the largest application of immunofluorescence is in identifying specific cell surface features thereby allowing the discrimination of lymphocyte subpopulations (phenotyping normal and malignant cells), for analysing complex receptors on neutrophils and monocytes, for detecting the appearance of new surface antigens induced by cell differentiation or activation. Besides this, the technique sets objective standards for appreciating cell activation (changes in RNA content, increased phagocytosis, DNA synthesis, changes in membrane fluidity, pH gradient changes). Computer processing of measurements allows rapid accurate studies of several properties simultaneously.