The persistence of immunogenicity of lipopolysaccharide antigens of E. coli and of sheep red blood cells in the mouse was studied by a transfer system. Immunized mice were lethally irradiated and re-populated with non-immune syngeneic lymphoid and bone marrow cells at different times after immunization. Persistence of immunogenicity in the irradiated hosts was revealed as the induction of a primary immune response in the transferred cells. In this system SRBC persisted in an immunogenic form for 14 days whereas the bacterial antigens remained immunogenic for at least 45 days.