The prevention of tolerance induction by a thymus‐derived (T) cell factor induced with a complex of synthetic polyadenylic‐polyuridylic acid (poly (A) · poly(U)) was studied. The soluble factor(s) was obtained by exposing mouse thymocytes to poly (A) · poly(U) for 10 min, washing and culturing them for an additional 6 h in serum‐free medium. This factor in the supernatant fluid of the culture, termed AUSF, prevented tolerance induction to soluble bovine IgG (sBGG) in vivo when administered to mice 3 h after tolerogen. The effect was manifested by a marked increase in rosette formation by spleen cells and hemagglutinin titers to BGG compared with mice given sBGG only. And, as shown previously, AUSF enhanced plaque formation to red blood cells (RBC) by spleen cells in vitro. Supernatant fluids from untreated, but cultured, thymocytes showed some activity, but much less than those from poly (A) poly(U)‐stimulated cultures. AUSF was found to function across histocompatibility differences since preparations from both syngeneic and allogeneic mice were effective in interfering with tolerance induction. The data suggest that poly(A) · poly(U) prevents tolerance induction to a thymus‐dependent antigen by the early release of a T cell factor before antigen‐reactive T cells have become tolerant.