Immunocompetent cells among mouse thymocytes: a minor population.

Suspensions prepared from thymuses of TL(+) mice contain a majority of TL(+) cells and a minority of TL(-) cells. The graft versus host reactivity of the TL(-) population is much greater than that of the whole population, as judged by the numbers of cells required to give splenic enlargements in Simonsen's assay. It is proposed that the TL(-) thymocyte represents a stage in the differentiation of TL(+) thymocytes into immunocompetent lymphocytes.