CD19 is present in both normal and malignant B-cells and has long been considered to be the most reliable surface marker of this lineage over a wide range of maturational stages. In normal lymphoid tissue, CD19 is observed in germinal centers, mantle zone cells, and scattered cells in the interfollicular areas. Anti-CD19 exhibits an overall immunoreactivity pattern similar to those of the antibodies against CD20 and CD22. However, in contrast to CD20, CD19 is also expressed in immature B-cells; although recent studies have described CD19 loss in a few B-cell neoplasms.^1-3