AbstractThe plaque‐forming cell (PFC) assay measures the humoral immune response mediated by the concerted actions of antigen‐presenting cells, T lymphocytes, and B lymphocytes. The most common form of the plaque method is used for the detection of murine primary IgM antibodies directed against the T cell–dependent sheep red blood cell (sRBC) surface antigens. Research has shown that the PFC response to sRBC is not only an excellent monitor of the primary effector function of the B cell, but that it may be the most sensitive immune parameter currently available to identify chemical perturbation. Several modifications and variations of the PFC assay are described in detail in this unit.