The rate of renewal of T lymphocytes in the bone marrow of euthymic C57BL/Ka and athymic nu/nu BALB/c mice was estimated by in vivo labeling with bromodeoxyuridine. T lymphocytes accounted for 16-18% of marrow cells in euthymic mice as judged by immunofluorescent staining with monoclonal antibodies for Thy-1, CD3, and alpha/beta T cell antigen receptor markers. About 70% of marrow cells expressed receptors (Mac-1, Gr-1, B220) for myeloid, macrophage, and B lineage cells. Approximately 13% of cells in the athymic bone marrow expressed alpha/beta T cell receptors. Sorted marrow T cells proliferated in response to stimulation with anti-alpha/beta antibodies in vitro and showed functional rearrangements of V beta and J beta genes. Sorted non-T cells did not respond to stimulation in vitro, and all V beta and J beta gene rearrangements identified were nonfunctional. In vivo labeling studies indicated that approximately 17 x 10(6) bone marrow T cells are renewed daily in euthymic mice and approximately 14 x 10(6) are renewed in athymic mice. Approximately 11 x 10(6) mature B cells (immunoglobulin M+) are renewed daily in the bone marrow of the latter mice. To determine whether marrow precursors can give rise to T cells directly, marrow cells from euthymic and athymic mice were depleted of T cells by cell sorting and incubated in vitro for 48 h in the absence of exogenous growth factors or thymic stromal cells. Examination of the cells after culture showed that 10-12% stained brightly for alpha/beta T cell receptors. Although functional rearrangements of V beta and J beta genes were not detected before culture, the majority of rearrangements were functional after culture. The emergence of the bright alpha/beta T cells in culture was dependent on depletion T cells from the marrow cells before culture. The results suggest that most marrow T cells are generated in the marrow itself.