New Zealand Black (NZB) mice spontaneously develop an autoimmune hemolytic anemia together with a markedly increased production of polyclonal antibodies. The spontaneous generation of anti-mouse red blood cells (MRBC), anti-bromelain-treated MRBC (BrMRBC) and anti-DNA autoantibodies was compared to the polyclonal antibody formation in irradiated (800 rad) 2-month-old NZB mice reconstituted with bone marrow cells (BMC) from 2- or 10-month-old NZB mice. The injection of 10-month-old NZB BMC markedly accelerated the mortality rate in parallel with the progressive increase of anti-MRBC and anti-BrMRBC autoantibody production, but the spontaneous production of polyclonal IgM antibodies and anti-DNA autoantibodies was completely abolished down to the levels of non-autoimmune mice. In contrast, mice reconstituted with 2-month-old NZB BMC exhibited neither the acceleration of anemia nor the lack of polyclonal antibody production. These results strongly suggest that the spontaneous production of anti-MRBC autoantibodies, including anti-BrMRBC autoantibodies, in the NZB mouse occurs independently of the polyclonal B cell activation, and that they result from a specific immune stimulation, while the anti-DNA autoantibody production is a consequence of polyclonal antibody formation.