Flow cytometric technology has evolved tremendously over the past 5 years. Among the major advancements have been the development of monoclonal antibodies, fluorescent labels, and computer software that makes it possible to perform routine multiparameter analyses. The clinical application of such analyses requires that standards and quality control procedures be established. Areas that need to be specifically addressed include sample preparation, staining, instrument calibration, and sample and data analysis. This study describes the basic elements involved in establishing such procedures and explores the use of multiparameter flow cytometric analysis in the characterization of peripheral blood mononuclear cells. Two-color, flow cytometric analysis of T, B, and natural killer cells can provide important insights into the biologic features of these cells as well as significant diagnostic information.