NK1. 1+ T cells in the mouse thymus and bone marrow were compared because some marrow NK1. 1+ T cells have been reported to be extrathymically derived. Almost all NK1. 1+ T cells in the thymus were depleted in the CD1−/−, β 2 m−/−, and J α 281−/− mice as compared with wild-type mice. CD8+ NK1. 1+ T cells were not clearly detected, even in the wild-type mice. In bone marrow from the wild-type mice, CD8+ NK1. 1+ T cells were easily detected, about twice as numerous as CD4+ NK1. 1+ T cells, and were similar in number to CD4− CD8− NK1. 1+ T cells. All three marrow NK1. 1+ T cell subsets were reduced about 4-fold in CD1−/− mice. No reduction was observed in CD8+ NK1. 1+ T cells in the bone marrow of J α 281−/− mice, but marrow CD8+ NK1. 1+ T cells were markedly depleted in β 2 m−/− mice. All NK1. 1+ T cell subsets in the marrow of wild-type mice produced high levels of IFN-γ, IL-4, and IL-10. Although the numbers of marrow CD4− CD8− NK1. 1+ T cells in β 2 m−/− and J α 281−/− mice were similar to those in wild-type mice, these cells had a Th1-like pattern (high IFN-γ, and low IL-4 and IL-10). In conclusion, the large majority of NK1. 1+ T cells in the bone marrow are CD1 dependent. Marrow NK1. 1+ T cells include CD8+, V α 14-J α 281−, and β 2 m-independent subsets that are not clearly detected in the thymus.