A significant proportion of cord blood CD5+ B cells express the activation molecules CD23, CD25, and transferrin receptor; react with the cell-cycle-associated monoclonal antibody (MoAb) Ki67; can be induced to enter the S phase of cell cycle by interleukin-2 (IL-2), IL-4, or low-molecular-weight B-cell growth fact or (lmw-BCGF) and, exposed to IL-1 and IL-2, acquire the features (slgD-, CD5-, CD10+, CD38+) of B blasts proliferating in the germinal centers of secondary follicles. These findings indicate that CD5+ B cells are preactivated and, in the proper microenvironment, may give rise to CD5- B cells.