Lymphocyte activation gene 3 (LAG‐3/CD223) is a CD4 homolog known to be selectively expressed in activated T and NK cells. It is thought to have a negative regulatory function in T cells. With the help of new monoclonal antibodies against mouse LAG‐3, we show that LAG‐3 surface expression is not limited to activated T and NK cells but is also found on activated B cells. Induction of B cell surface expression is T cell dependent and mediated by a soluble factor. The majority of LAG‐3 on B cell surface is endogenously produced, even though soluble LAG‐3 is present in the culture supernatants and can be passively absorbed. As B cells express LAG‐3 in a T cell dependent manner and not when activated by Toll‐like‐receptor agonists alone, we propose LAG‐3 as a new marker of T cell induced B cell activation.