AbstractMRL/Mp‐lpr/lpr(MRL‐lpr) mice develop a generalized lymphadenopathy reflecting the expansion of a Lyt‐2−T cell population. The present report describes the pattern of reactivity of a xenogeneic monoclonal antibody (mAb 100C5) directed against this T cell population. By single‐ and two‐color flow cytofluorometry analysis this mAb was found to stain brightly 80–90% of T cells in enlarged MRL‐lpr lymph nodes. In contrast lymph node T cells from congenic MRL‐MP‐+/+ (MRL‐+) mice were either unstained (70%) or weakly stained (30%), these latter cells being mostly Lyt‐2+. Unexpectedly, all lymph node B cells from MRL‐+ or MRL‐lpr mice were as strongly 100C5+as MRL‐lpr T cells. Similar observations were made in C57BL/6‐lpr/lprand C57BL/6‐+/+ mice. Molecular weight determination suggested that the 100C5 mAb binds to the same molecule (Mr= 220000) on M