Expression of transgene-encoded proteins in the pancreatic islets can cause peripheral deletion of T cells. However, tolerance has not been observed in all transgenic models. It has been proposed that the determining factor for successful peripheral tolerance is the amount of Ag cross-presented by quiescent APCs. Using InsHA mice, which demonstrate peripheral tolerance to the influenza virus hemagglutinin (HA) expressed in the pancreatic islet β cells, we have investigated the consequences when different amounts of HA are expressed. As compared with InsHA mice that are heterozygous for the InsHA transgene, homozygous InsHA mice demonstrated enhanced activation and proliferation of K d-restricted HA-specific CD8+ T cells in the pancreatic lymph nodes. However, despite such activation, insulitis was not observed, and the T cells were gradually functionally deleted. Deletion of these activated cells occurred much more rapidly in homozygous than in heterozygous InsHA mice. These data demonstrate that there is a direct correlation between the amount of HA expressed in the periphery, and both the degree of T cell proliferation in the pancreatic lymph nodes and the rate of tolerance of HA-specific CD8+ T cells. This strongly supports the hypothesis that activation of T cells through cross-presentation of peripheral Ags in a noninflammatory environment is an important part of the normal mechanism of tolerance to Ags expressed in the pancreatic islets.