Abstract: A biologically active composition extracted from thymus tissue, capable of inducing immature bone marrow cells to differentiate into competent suppressor T-cells.

Abstract: This invention relates to a new composition of matter, and more particularly to a novel thymic factor isolated in purified form from mammalian thymus tissue which induces immature bone marrow cells to differentiate into competent suppressor T-cells (T.sub.s). This novel thymic factor, which can also be called inducer of T-suppressor cells (iT.sub.s), has a molecular weight of approximately 65,000 daltons, as determined by gel filtration, a pI of 4.2-4.5, and is heat stable up to about 80.degree. C. iT.sub.s induces immature bone marrow cells to differentiate into cells which exhibit a phenotypic marker of suppressor T-cells and which when adoptively transferred repress reconstitution of the immune response in irradiated, thymectomized mice. An antibody has also been raised to iT.sub.s. This invention also relates to therapeutic methods and fields of use for iT.sub.s.