Reacts with a carbohydrate determinant, identified as CDw78 (Workshop V; Code CDw78.2). Epitope recognized by this MAb and Leu21 is the same. It is present on some immature and some peripheral blood B cells, and some B cell lines. Its expression is increased after B-cell activation in vitro. CDw78 is also found on tissue macrophages and on epithelial cells, but not on T cells, NK cells, monocytes, granulocytes and myeloid tissues. It appears that CDw78 is a determinant on a major histocompatibility complex (MHC) class II subpopulation.