The precursor of the pokeweed mitogen (PWM)-induced anti-sheep red blood cell (SRBC) plaque-forming cell (PFC) in human peripheral blood was characterized. By a variety of purification procedures, it was demonstrated to be a lymphocyte with surface characteristics of a B cell. Furthermore, it was demonstrated to bind to sheep erythrocytes (E) and thus segregated with the E-rosetting T cells when T cell enrichment was performed by differential fractionation of E-rosetting cells. This binding of the PFC precursor to E was blocked by pretreating the lymphocyte with anti-human Ig before E rosetting, indicating that the PFC precursor specifically bound to SRBC by a surface Ig molecule with binding specificity for sheep red blood cell determinants. Hence, the precursor of the PWM-triggered anti-SRBC PFC is a B lymphocyte with surface Ig expressing specificity for SRBC.