The role of thymus-influenced antigen reactive cells (ARC) in the immune response was studied with the aid of a potent adjuvant to both antibody formation and cell-mediated immunity, polyadenylic-polyuridylic acid complexes (poly A:U). The polynucleotide complex increased the anti-sheep erythrocyte rosette forming cell response of irradiated mice after thymus cells were exposed to poly A:U in vivo or in vitro prior to injection of bone marrow cells. This direct stimulatory action resulted in an enhancement of the rate of proliferation of ARC in response to antigen and a shortening of the interval between antigenic challenge and the initiation of division in immunocompetent cells. No stimulatory effect of poly A:U on bone marrow derived antibody forming precursor cells was observed. It is suggested that ARC play an important role in the regulation of the length of the induction period, as well as the rate of increase of antibody forming cells in response to some antigenic stimuli.