The serum of partially hepatectomized rabbit taken 4 h after operation possessed the proerty of enhancing the immune response to sheep red blood cells (SRBC) in homologous recipients. An exhaustive absorption of the serum with SRBC stromata did not affect its adjuvant-like activity indicating that an active factor augmented the immune response non-specifically. It was shown that the serum of partially hepatectomized rabbits contained a 5-2S IgG fragment antigenically related to peptic F(ab')2 fragment. The i.v. injection of this fragment together with SRBC into rabbits resulted in enhancement of PFC response and haemagglutinin production. Similar enhancing effect was produced by peptic F(ab')2 fragment obtained from rabbit IgG devoid of anti-SRBC antibodies. Sera of partially hepatectomized rabbits did not contain F(ab')2-like fragments and failed to enhance the immune response if the animals were treated with proteinase inhibitor ('Trasylol'), indicating that the fragment was a cetabolic product of IgG. The results are interpreted in terms of regulation of immunoglobulin synthesis by a split product of autologous IgG.