Purified human B cells were activated in cultures in the absence of any intentional stimulants as judged by lymphokine synthesis and proliferation. These responses were not augmented by monocytes. Lymphokine production (LIF) was increased in the presence of T cells. Autologous mixed lymphocyte reaction of T cells against B cells (AMLR) did not include LIF production in spite of the proliferative response. We would suggest that activated B cells present in the population of purified B cells are the stimulators in AMLR. In our interpretation these results support the hypothesis that AMLR reflects a mechanism by which T cells regulate lymphocyte function.