Circulating IgD⁺ CD38^hi B cells express immature B-cell markers with an overall phenotype similar to transitional type I (T1) B cells. B cells were enriched from blood of healthy adult donors and stained with anti-CD19, anti-IgD, anti-CD38, and a fourth anti–human mAb. The differential expression of a variety of B-cell markers was compared between the IgD⁺ CD38^hi immature population (solid line) and IgD⁺ CD38⁺ mature naive B cells (dashed line, indicated with arrow). Results are representative of data from at least 4 healthy adult donors.