This panel was developed to characterize the phenotypic diversity of human memory B cells, with an emphasis on discriminating cell subsets within both the conventional memory population (CD27 1 ) and the more recently described isotype switched (IgD 2 ) population lacking expression of CD27 (1). It has been tested on fresh and cryopreserved peripheral blood mononuclear cells (PBMC), as well as bone marrow aspirates and tonsillar cells (Table 1). The multicolor panel described herein has been used extensively to analyze large numbers of PBMC samples obtained from healthy controls in steady state and in response to infection (HIV, influenza, respiratory syncytical virus) and vaccination (influenza, tetanus) as well as in hundreds of patients with autoimmune diseases (systemic lupus erythematosus (SLE), rheumatoid arthritis, Sjogren's syndrome, psoriatic arthritis and Type 1 diabetes) and conditions characterized by allogeneic immune responses (renal transplantion and chronic graft versus host disease). This panel is also being applied in a longitudinal study in which 150 SLE patients are to be followed quarterly for a period of 2 years.