We have previously shown that in the nonobese diabetic (NOD) mouse, an experimental model for autoimmune insulin-dependent diabetes, spleen diabetogenic T cells are contained within the T-cell subpopulation that express no or low levels of L-selectin. This phenotype characterizes activated/memory T cells. In the present study, we have compared the distribution of autoreactive T cells to that of L-selectin -/lo T cells in prediabetic and diabetic mice. Activated/memory T cells were found in decreasing concentrations in the bone marrow (BM), spleen, peritoneum, lymph nodes, and blood. This distribution correlated perfectly with that of T cells capable of transferring diabetes into syngeneic nondiabetic recipients. In diabetic mice, the highest levels of diabetogenic cells were observed in the spleen and BM. The peritoneum and lymph nodes contained intermediate frequencies of autoreactive cells. In the peripheral blood, the number of autoreactive T cells was variable, usually low; in some cases, they were undetectable. These cells were rare in the thymus. Diabetes-transferring cells were present in the spleen and BM of prediabetic mice. In these animals, diabetogenic cells were present in the blood circulation with frequencies higher than in diabetic mice, suggesting that after disease onset, when almost all target beta cells have disappeared, the recirculation of autoreactive cells is greatly decreased and finally stops. These observations: (a) suggest that T cells from BM of prediabetic and diabetic patients may be a better source of diabetogenic cells than the blood for analyzing diabetes-associated responses or for generating diabetogenic T-cell clones, and (b) point up the risk of using healthy autoimmune-prone donors for BM transplantation.