A crude cell wall-rich fraction of Listeria monocytogenes contains a substance or substances which cause blast transformation and thymidine incorporation by mouse spleen cells. The substance, Listeria cell wall fraction (LCWF), is a B cell mitogen in the mouse. It is not endotoxin, in that it stimulates B cells in C3H/HeJ mice. It is weakly mitogenic for rat spleen cells and not at all mitogenic for human peripheral blood or tonsil lymphocytes. Thus, it is dissimilar to both endotoxin and purified protein derivative (PPD) in its spectrum of activity. A cell wall-associated mitogen may play an early role in Listeria resistance.